 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued January 11, 2017              Decided August 4, 2017

                       No. 15-5332

 AMERICAN WILD HORSE PRESERVATION CAMPAIGN, ET AL.,
                   APPELLANTS

                             v.

     SONNY PERDUE, SECRETARY, U.S. DEPARTMENT OF
                AGRICULTURE, ET AL.,
                      APPELLEES


        Appeal from the United States District Court
                for the District of Columbia
                    (No. 1:14-cv-00485)


    David Zaft argued the cause for appellants. With him on
the briefs was William S. Eubanks II. Katherine A. Meyer
entered an appearance.

     Mark R. Haag, Attorney, U.S. Department of Justice,
argued the cause for appellees. With him on the brief were
John C. Cruden, Assistant Attorney General at the time the
brief was filed, Meredith L. Flax and Stuart Gillespie,
Attorneys, U.S. Department of Justice, and Steven F. Hirsch,
Attorney-Advisor, U.S. Department of Agriculture.

     Caroline Lobdell was on the brief for defendants-
intervenors-appellees.
                                2

    Before: TATEL, MILLETT, and WILKINS, Circuit Judges.

    Opinion for the Court filed by Circuit Judge MILLETT.

     MILLETT, Circuit Judge: Since 1975, the United States
Forest Service has protected and managed wild horses in the
Devil’s Garden section of the Modoc National Forest in
Northern California. That wild horse territory originally
consisted of two separate tracts of land of roughly 236,000
acres. But at some point in the 1980s, a Forest Service map
added in an approximately 23,000 acre tract of land known as
the Middle Section and, in so doing, linked the two territories
into a larger and unified wild horse territory of approximately
258,000 acres. For more than two decades, the Service
continued to describe the territory as a single contiguous area
and to manage wild horses in the Middle Section.

     In 2013, the Forest Service publicly acknowledged the
cartographic confusion, declared the expansion reflected in the
1980s map to be an administrative error, and without further
analysis redrew the wild horse territory’s lines to exclude the
Middle Section and to revert to two disjoined tracts of land.
The American Wild Horse Preservation Campaign and other
plaintiffs filed suit alleging that the Service’s revamping of the
territorial lines violated numerous federal laws. We agree. A
23,000 acre tract of land and two decades of agency
management cannot be swept under the rug as a mere
administrative mistake. We accordingly reverse in part and
remand for the Service to address rather than to ignore the
relevant history.

                                    I

     The Modoc National Forest comprises approximately 1.6
million acres of federally managed land in Northern California.
                               3
Included within the Forest are several hundred thousand acres
of protected wild horse land that make up the Devil’s Garden
Wild Horse Territory. The Forest Service’s management of the
Devil’s Garden Wild Horse Territory is subject to a
Matryoshka doll of nesting federal statutes.

     First, the Wild and Free-Roaming Horses and Burros Act
of 1971 (“Wild Horses Act”), 16 U.S.C. § 1331 et seq., charges
the Secretaries of Interior and Agriculture with “protect[ing]
and manag[ing] wild free-roaming horses and burros” on
federal lands, id. § 1333(a). The Secretaries “may designate
and maintain specific ranges on public lands as sanctuaries for
their protection and preservation,” and “shall manage wild
free-roaming horses and burros in a manner that is designed to
achieve and maintain a thriving natural ecological balance[.]”
Id. The Secretaries also “shall maintain a current inventory”
and set “appropriate management levels” for “wild free-
roaming horses and burros,” to ensure a “thriving natural
ecological balance” and to “protect the range from the
deterioration associated with overpopulation.”              Id.
§ 1333(b)(1), (2). Wild horses are to be treated “in the area
where presently found” as an integral component “of the
natural system of the public lands.” Id. § 1331.

     The Service is responsible for implementing the Wild
Horses Act within the National Forest System. 36 C.F.R.
§ 222.60(a). In 1980, the Service promulgated regulations
providing that it “shall: * * * [e]stablish wild horse and burro
territories” (“Wild Horse Territories”), and then “[a]nalyze,”
“develop[,] and implement a management plan” for each Wild
Horse Territory. Id. § 222.61(a)(3)–(4). The Service may
“update[]” the Wild Horse Territory Plans “whenever needed,
as determined by conditions on each territory.”              Id.
§ 222.61(a)(4). The Service must also “[m]aintain a current
inventory of [wild horses] on each [Wild Horse Territory] to
                                 4
determine * * * where     excess      animals     exist[,]” id.
§ 222.61(a)(5), set “appropriate management levels” for those
horses and burros, and “remov[e] or destr[oy] * * * excess
animals,” id. § 222.61(a)(6); see also id. § 222.69.

    Service regulations further define “[w]ild free-roaming
horses and burros” to mean “all unbranded and unclaimed
horses and burros and their progeny that” either “have used
lands of the National Forest System on or after December 15,
1971,” or that “do hereafter use these lands as all or part of their
habitat.” 36 C.F.R. § 222.60(b)(13). Those animals retain
federal protection even if they “move to lands of other
ownership or jurisdiction as a part of their annual territorial
habitat pattern or for other reasons.” Id. § 222.65.

     Second, the National Forest Management Act of 1976
(“Forest Management Act”), 16 U.S.C. § 1600 et seq.,
“requires the Secretary of Agriculture to ‘develop, maintain,
and, as appropriate, revise land and resource management plans
for units of the National Forest System.’” Ohio Forestry Ass’n
v. Sierra Club, 523 U.S. 726, 728 (1998) (quoting 16 U.S.C.
§ 1604(a)). The Secretary has delegated his authority under the
Act to the Service. 36 C.F.R. § 200.3(b).

    The Forest Management Act establishes a two-step
procedure for managing National Forest System lands. The
Service must (i) “develop, maintain, and, as appropriate, revise
land and resource management plans” for national forests
(“Forest Plans”), and (ii) ensure that all “[r]esource plans and
permits, contracts, and other instruments for the use and
occupancy of National Forest System lands,” including Wild
Horse Territory Plans, are “consistent with the [Forest Plans].”
16 U.S.C. § 1604(a), (i).

    The Forest Management Act sets out several general
conditions with which the development of Forest Plans must
                              5
comply. See 16 U.S.C. § 1604(f). For instance, the Service
must “provide for public participation in the development,
review, and revision of [Forest Plans].” Id. § 1604(d). In
addition, the Plans must “be embodied in appropriate written
material, including maps and other descriptive documents,” id.
§ 1604(f)(2), and “be prepared by an interdisciplinary team,”
id. § 1604(f)(3). The Forest Service may amend Forest Plans
“in any manner whatsoever after final adoption[.]” Montanans
for Multiple Use v. Barbouletos, 568 F.3d 225, 227 (D.C. Cir.
2009) (quoting 16 U.S.C. § 1604(f)(4)); see also 36 C.F.R.
§ 219.13(a). But if an “amendment” of a Forest Plan “would
result in a significant change,” the amendment process must
comply with heightened procedural requirements. See 16
U.S.C. § 1604(f)(4). Regardless of whether an amendment is
significant, however, the Forest Service must allow for public
participation in the amendment process. Id.

     Service regulations elaborate upon the procedures for
developing and amending Forest Plans. 36 C.F.R. §§ 219.1–
219.19. As relevant here, the Service must develop Forest
Plans in coordination with the statutorily required
interdisciplinary team, extensive public participation and
comment, and related efforts of other federal agencies, state
and local governments, and Indian tribes. Id. §§ 219.4, 219.5.
Formulation of such Plans must take into consideration, inter
alia, “fish and wildlife species,” “grazing and rangelands,”
“habitat and habitat connectivity,” “[h]abitat conditions,” and
“[l]and status and ownership, use, and access patterns relevant
to the [Forest Plan] area.” Id. § 219.10(a). The Plan must also
“maintain the diversity of plant and animal communities”
within the forest. Id. § 219.9.

    Third, the National Environmental Policy Act (“NEPA”),
42 U.S.C. § 4321 et seq., obligates federal agencies to analyze
the environmental consequences of proposed federal actions.
                               6
See generally id. § 4332. Under NEPA, federal agencies must
conduct an Environmental Assessment to determine whether a
proposed federal action will have a significant effect on the
environment. 40 C.F.R. §§ 1508.9(a), 1508.13. If that
Assessment indicates that the environmental impacts will not
be significant, the agency must issue a “finding of no
significant impact,” id. § 1501.4(e), explaining why the agency
action will not substantially affect the environment, id.
§ 1508.13. But if the Assessment indicates that the proposed
action will “significantly affect[] the quality of the human
environment,” the agency must prepare an Environmental
Impact Statement detailing: “(i) the environmental impact of
the proposed action, (ii) any adverse environmental effects
which cannot be avoided should the proposal be implemented,
(iii) alternatives to the proposed action, (iv) the relationship
between local short-term uses of [the] environment and the
maintenance and enhancement of long-term productivity, and
(v) any irreversible and irretrievable commitments of resources
which would be involved in the proposed action should it be
implemented.” 42 U.S.C. § 4332(C); see also 40 C.F.R.
§ 1502.2.

     Fourth, the Administrative Procedure Act, 5 U.S.C. § 551
et seq., prohibits arbitrary and capricious actions by federal
agencies and mandates that they give reasoned explanation for
the actions that they do take. See, e.g., Motor Vehicle Mfrs.
Ass’n v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 42–52
(1983); see also Public Citizen, Inc. v. FAA, 988 F.2d 186, 197
(D.C. Cir. 1993) (“The requirement that agency action not be
arbitrary or capricious includes a requirement that the agency
adequately explain its result[.]”).
                               7
                              II

    While this dispute concerns a 2013 decision by the Service
to change the boundaries of the Devil’s Garden Wild Horse
Territory, the origins of the controversy reach back four
decades.

                               A

     In 1975, the Service issued its first Devil’s Garden Wild
Horse Territory Plan. The Wild Horse Territory specified in
that plan consisted of two separate areas of land totaling
approximately 236,000 acres. The Territory did not include a
parcel of land of approximately 23,000 acres, known as the
Middle Section, which conjoined those two separate tracts.

     Sometime in the 1980s, a Forest Service map depicted the
Devil’s Garden Wild Horse Territory as a single contiguous
area of land that included the Middle Section. According to the
Service, the map’s “revised boundary” “incorporated about
another 23,631 acres of land,” resulting in a Wild Horse
Territory of “approximately 258,000 acres in size.” J.A. 261.

     Then, in 1991, the Service issued a Forest Plan for the
Modoc National Forest. While the Plan did not include a map
of the Wild Horse Territory, the Forest Plan acknowledged that
the Service “is legally obligated to manage horses within a
258,000-acre wild horse territory,” J.A. 584, and announced
that “[t]he Forest has one wild horse territory of about 258,000
acres,” J.A. 585.

    The 1991 Forest Plan also stated that the Service “prepared
the Wild Horse [Territory] Plan in 1985, which identifies a
population objective of 275–335 animals to manage.” J.A.
                                  8
585–586. 1 The interdisciplinary team that prepared the 1991
Forest Plan expressly denominated the “Wild Horse [Territory]
Plan” to be “consistent with, and still appropriate for, the
[1991] Forest Plan.” J.A. 578. As a result, the Wild Horse
Territory Plan, which the interdisciplinary team understood to
include a single 258,000 acre territory, was expressly
“incorporated by reference” into the 1991 Forest Plan. J.A.
578; see also J.A. 588 (describing “Wild Horse [Territory]
Plan” as an “[e]xisting [p]lan[] [r]etained and [i]ncorporated by
[r]eference into the [1991] Forest Plan and [u]pdated to be
[c]onsistent”) (emphasis added). When the 1991 Forest Plan
was finally approved, it “supersede[d] most previous Forest
resource management plans.” J.A. 578.

    Over the next two decades, the Service actively managed
and recorded wild horses in the Middle Section, as evidenced
by official Wild Horse Inventory Reports from that time period.

                                  B

     The Modoc National Forest is divided into grazing
“allotments.” Generally speaking, the Wild Horse Territory
boundaries do not hew precisely to those of the grazing
allotments. Rather, the Wild Horse Territory covers portions
of various allotments. The disputed Middle Section consists of
portions of five allotments: the Triangle, Avanzino, Carr,
Timbered Mountain, and Big Sage Allotments. In 1971, when
the Wild Horses Act was adopted, two portions of allotments
in the Middle Section were privately held: the Triangle
portion, and the Avanzino portion. In total, those private lands
covered approximately 5,923 acres. The other portions—the
Carr, Timbered Mountain, and Big Sage portions—were
publicly held in 1971. Consequently, as stipulated by the

    1
        The Service denies that there was a 1985 Wild Horse Territory
Plan.
                               9
Service, the “majority of the lands in the [Middle Section] were
publicly held in 1971[.]” J.A. 66.

    In 1976, the Service acquired the Triangle Allotment as
public land. That means that, in 1991, when the Service
adopted the Forest Plan that included the Middle Section in the
Wild Horse Territory, only one portion of the Middle Section
was privately held: the Avanzino portion. That private land
covered approximately 10% of the Middle Section’s acreage.

                               C

     In July 2011, the Service issued a scoping letter proposing
to update the Devil’s Garden Wild Horse Territory Plan. That
letter indicated that the Wild Horse Territory was
“approximately 268,750 acres in size,” J.A. 757, and included
a map depicting the Wild Horse Territory as a single,
contiguous area of land including the Middle Section.

     A year later, the Service turned its attention to the
discrepancy between the Wild Horse Territory boundaries in
the 1991 Forest Plan and the boundaries in the original 1975
Wild Horse Territory Plan. The Forest Service issued a new
scoping letter in December 2012 stating that, “[d]uring the mid-
1980’s, the [Modoc National Forest’s Devil’s Garden and
Doublehead Ranger Districts] appear[] to have adjusted the
[Wild Horse Territory] boundary for administrative
convenience” to “incorporate[] about [an additional] 23,631
acres of land.” J.A. 731. The December 2012 scoping letter
called this an “administrative error” and “propose[d] to return
to the management of wild horses within the [Wild Horse
Territory] boundary established in 1975.” J.A. 732.

     In August 2013, the Service released a Final
Environmental Assessment to accompany its proposed
revisions to the size of the Wild Horse Territory. Like the
                              10
scoping letter, the Final Environmental Assessment labeled the
inclusion of the Middle Section “[a]n administrative error,”
J.A. 264, 708, and “propose[d] to return to the management of
wild horses within the [Wild Horse Territory] boundary” as
originally established, J.A. 264, 710. The Service also issued
a Decision Notice and Finding of No Significant Impact,
incorporating the Final Environmental Assessment by
reference. That Decision formally adopted the proposed action
set forth in the Final Environmental Assessment and concluded
that contracting the Wild Horse Territory as proposed would
not have a sufficiently significant environmental impact to
necessitate an Environmental Impact Statement.

    Also in August 2013, the Service issued its new Devil’s
Garden Wild Horse Territory Plan. The 2013 Wild Horse
Territory Plan indicated that the boundaries of the Wild Horse
Territory would mirror those of the 1975 Wild Horse Territory
Plan, which did not include the disputed Middle Section.

                              D

    After exhausting their administrative remedies, plaintiffs,
the American Wild Horse Preservation Campaign, Return to
Freedom, and Carla Bowers (collectively, “Campaign”), all of
which advocate for the protection of wild horses, filed suit in
the United States District Court for the District of Columbia.
They contend that the Service’s 2013 revision of the Devil’s
Garden Wild Horse Territory violated the Administrative
Procedure Act, the Wild Horses Act, the Forest Management
Act, and NEPA. See American Wild Horse Preservation
                                 11
Campaign v. Vilsack, No. 14-0485, Docket Entry No. 1
(D.D.C. Mar. 24, 2014) (Compl. ¶¶ 58–91). 2

     On September 30, 2015, the district court granted
summary judgment for the Service. American Wild Horse
Preservation Campaign v. Vilsack, 133 F. Supp. 3d 200
(D.D.C. 2015). The court concluded that the Service
permissibly found that the Middle Section was never
incorporated into the Wild Horse Territory, and that any
reference to a single, contiguous Wild Horse Territory was a
mere “administrative error.” Id. at 212; see also id. at 212–218.
The district court also ruled that the Service’s redrawing of the
boundaries of the Wild Horse Territory did not amount to a
“significant” amendment warranting formal procedures under
the Forest Management Act, as the Service was already
managing the Wild Horse Territory as two noncontiguous
units. Id. at 219–220. The district court similarly reasoned
that, because the Service’s “boundary adjustment simply
corrected an administrative error and resulted in the continued
management of the [Middle Section] as distinct from the [Wild
Horse Territory],” the Service reasonably determined that its
boundary correction would not significantly affect the quality
of the human environment, within the meaning of NEPA. Id.
at 221.

                                III

     We review the district court’s grant of summary judgment
de novo. Theodore Roosevelt Conservation P’ship v. Salazar,
661 F.3d 66, 72 (D.C. Cir. 2011). Under the Administrative
Procedure Act, we must set aside the Service’s actions if they
are “arbitrary, capricious, an abuse of discretion, or otherwise

    2
        The Campaign also originally challenged the management
level for horses set by the Service, but does not press that issue on
appeal.
                               12
not in accordance with law.” 5 U.S.C. § 706(2)(A). That
standard obligates the agency to examine all relevant factors
and record evidence, and to articulate a reasoned explanation
for its decision. See State Farm, 463 U.S. at 52. Generally, a
rule is arbitrary and capricious if the agency: “(1) ‘has relied
on factors which Congress has not intended it to consider,’ (2)
‘entirely failed to consider an important aspect of the problem,’
(3) ‘offered an explanation for its decision that runs counter to
the evidence before the agency,’ or (4) [offers an explanation
that] ‘is so implausible that it could not be ascribed to a
difference in view or the product of agency expertise.’” United
States Sugar Corp. v. EPA, 830 F.3d 579, 606 (D.C. Cir. 2016)
(quoting State Farm, 463 U.S. at 43).

    We hold that the Service’s decision to eliminate the
Middle Section of the Devil’s Garden Wild Horse Territory
Plan was arbitrary and capricious in two respects. First, the
Service failed to acknowledge and adequately explain its
change in policy regarding the management of wild horses in
the Middle Section as part of a single, contiguous protected
Wild Horse Territory. Second, the Service failed to consider
adequately whether an Environmental Impact Statement was
required under NEPA. Accordingly, we reverse the district
court’s grant of summary judgment for the Service.

                               A

     A central principle of administrative law is that, when an
agency decides to depart from decades-long past practices and
official policies, the agency must at a minimum acknowledge
the change and offer a reasoned explanation for it. See Encino
Motorcars, LLC v. Navarro, 136 S. Ct. 2117, 2126 (2016)
(“[A]n ‘[u]nexplained inconsistency’ in agency policy is ‘a
reason for holding an interpretation to be an arbitrary and
capricious change from agency practice[.]’”) (second alteration
                                 13
in original) (quoting National Cable & Telecomms. Ass’n. v.
Brand X Internet Servs., 545 U.S. 967, 981 (2005)); Lone
Mountain Processing, Inc. v. Secretary of Labor, 709 F.3d
1161, 1164 (D.C. Cir. 2013) (“[A]n agency changing its course
must supply a reasoned analysis indicating that prior policies
and standards are being deliberately changed, not casually
ignored. Failing to supply such analysis renders the agency’s
action arbitrary and capricious.”) (internal quotation marks and
citation omitted); United Mun. Distribs. Grp. v. FERC, 732
F.2d 202, 210 (D.C. Cir. 1984) (“[A]gencies must give a
reasoned analysis for departures from prior agency practice[.]”)
(citing Greater Boston Television Corp. v. FCC, 444 F.2d 841,
852 (D.C. Cir. 1970)). 3

     For that reason, we have long held that “[a]n agency may
not * * * depart from a prior policy sub silentio[.]” United
States Telecom Ass’n v. FCC, 825 F.3d 674, 707 (D.C. Cir.
2016) (quoting FCC v. Fox Television Stations, Inc., 556 U.S.
502, 515 (2009)); Verizon v. FCC, 740 F.3d 623, 636 (D.C. Cir.
2014) (same); Comcast Corp. v. FCC, 600 F.3d 642, 659 (D.C.
Cir. 2010) (same).


    3
       See also Mistick PBT v. Chao, 440 F.3d 503, 512 (D.C. Cir.
2006) (“Where an agency departs from established precedent without
a reasoned explanation, its decision will be vacated as arbitrary and
capricious.”) (quoting Ramaprakash v. FAA, 346 F.3d 1121, 1130
(D.C. Cir. 2003)); Nuclear Energy Inst., Inc. v. EPA, 373 F.3d 1251,
1296 (D.C. Cir. 2004) (“If an agency decides to change course * * *
we require it to supply a reasoned analysis indicating that prior
policies and standards are being deliberately changed, not casually
ignored.”) (internal quotation marks and citation omitted); National
Classification Comm. v. United States, 779 F.2d 687, 696 (D.C. Cir.
1985) (“[A]n agency may depart from past policies or practices if the
agency also provides a reasoned explanation for its actions.”)
(emphasis added).
                               14
     The Service’s main defense in this case, however, has been
to insist that nothing changed in 2013. In the Service’s view,
the Middle Section was never part of the Devil’s Garden Wild
Horse Territory, and so there was nothing to change. That
argument flatly defies the plain text of the official 1991 Forest
Plan, repeated official agency statements, and two decades of
agency practice. Blinders may work for horses, but they are no
good for administrative agencies.

    The Service argues secondly that the inclusion of the
Middle Section must be ignored because it lacked the legal
authority to add it in the mid-1980s. That argument never even
leaves the starting gate.

                               1

     The Service tries to shrug off its inclusion of the Middle
Section in the Wild Horse Territory as some sort of
inconsequential and passing “administrative error,” as though
that label nullifies any agency duty to reasonably explain its
about-face. But there is no “oops” exception to the duty of
federal agencies to engage in reasoned decisionmaking.
Accordingly, the Service’s decision runs aground on both the
facts and the law.

    As a matter of factual reality, this case involves far more
than an errant map. The Service’s inclusion of the Middle
Section in the Wild Horse Territory is well documented in the
administrative record, and it was reconfirmed repeatedly by
two decades of agency practice and official pronouncements.

    Most significantly, the official 1991 Forest Plan formally
documented that the Modoc National Forest “has one wild
horse territory of about 258,000 acres,” J.A. 585, and that the
Service “is legally obligated to manage horses within [that]
258,000-acre wild horse territory,” J.A. 584. That language
                              15
was no divagation. The Forest Plan was the product of more
than ten years of formal study by the Service involving
participation by: (i) hundreds of members of the public who
submitted numerous comments and gave 105 testimonials at
almost 50 public meetings, (ii) five other federal government
agencies, (iii) eleven state agencies, (iv) various local
governments, agencies, and elected officials; (v) four tribal
communities, and (vi) various industry and non-profit groups.
Such extensive study and wide participation renders fanciful
the notion that 23,000 acres of wild horse territory simply
slipped in unnoticed.

     The Service notes that the record contains no affirmative
indications that the Service “intended to expand the boundaries
of the existing Territory when it adopted the [1991] Forest
Plan.” Service’s Br. 16. True. But that is because the Forest
Plan treated the determination of the Wild Horse Territory’s
size as a decision that had already been made in an earlier
Service plan. See J.A. 585–586 (“The [Modoc National] Forest
has one wild horse territory of about 258,000 acres * * * *
Fulfilling requirements of the [Wild Horse] Act the [Service]
prepared the Wild Horse Management Plan in 1985[.]”). To be
sure, the Service denies that there was such a 1985 plan.
Service’s Br. 15 n.7. But that misses the point: The 1991
Forest Plan that the Service drafted, and in the formulation of
which it was the key player, described the Wild Horse
Territory’s expanded 258,000 acreage as a fait accompli by
1991. So the absence of expansion talk in the Forest Plan is no
surprise.

     What the 1991 Forest Plan did do was formally document
a single, contiguous, 258,000 acre Wild Horse Territory that
could only exist through inclusion of the Middle Section,
incorporate that status into the Plan through a notice-and-
comment process, and set a herd management level within that
                               16
territory of “275–335 animals to manage,” J.A. 586. In
addition, the Forest Plan’s explicit description of the size and
management levels for the Wild Horse Territory largely
repudiates the Service’s claims that the plan was “of no
practical consequence for the management of the disputed area
because the Forest Service never set appropriate management
levels for horses on the [portions of allotments] within the
disputed area.” Service’s Br. 44.

     The Service also argues that it did not intend to expand the
territory. But after-the-fact claims about agency intentions do
not work when agency actions evince the opposite. And the
Service’s actions for at least twenty years corroborated the
Middle Section’s inclusion in the Wild Horse Territory.

     More specifically, Wild Horse Inventory Reports for the
Devil’s Garden Wild Horse Territory documented the
Service’s treatment of portions of the Middle Section as part of
the Territory after the 1991 Forest Plan. In 1992 and 1993, the
Wild Horse Inventory Reports counted the “total [number of]
horses[,] mules & burros” in ten regions within the Modoc
National Forest. J.A. 859, 861. Big Sage, which is partially
contained within the Middle Section, and Boles Meadow,
which is entirely within the Middle Section, were both
excluded from the original 1975 Wild Horse Territory
boundaries. But they are listed as two of the ten main regions
that Service observers surveyed for wild horses in 1992 and
1993. The 1992 Wild Horse Inventory Report documented 33
horses in Big Sage and Boles Meadow combined—more than
ten percent of the total headcount of 321 horses for the entire
Wild Horse Territory. The 1993 Wild Horse Inventory Report
documented five horses in Big Sage and Boles Meadow
combined, out of 58 horses in total.
                              17
      In addition, statements accompanying the 1992 and 1993
Wild Horse Inventory Reports implicitly treated Big Sage and
Boles Meadow as within the Wild Horse Territory. For
example, in the 1992 Wild Horse Inventory Report, the Service
noted that some of the wild horses spotted were located outside
the Wild Horse Territory. See, e.g., J.A. 862 (1992 Wild Horse
Inventory Report: “Ten horses were observed in the West
Grizzlie * * * Allotment. This area is outside of the wild horse
territory. Every effort should be made to return these animals
to the territory and/or [have them] removed.”). By implication,
the remaining wild horses that the Service counted—including
those wild horses in Big Sage and Boles Meadow—were
considered to be inside the Wild Horse Territory. Moreover,
the 1993 Wild Horse Inventory Report recommended that
certain wild horses found outside the Wild Horse Territory be
relocated to Boles Meadow. J.A. 860 (1993 Wild Horse
Inventory Report: “There are 3 [wild horses] in the Blue Mtn.
Allotment that need to be relocated to the Boles [Meadow].
The Blue Mtn. Allotment is outside of the Territory.”)
(emphasis added). That has to mean that the Service
considered Boles Meadow (in the Middle Section) to be within
the Wild Horse Territory. Why else would the Service devote
scarce resources to shuffling horses from one location outside
the Territory to another one also outside the Territory?

    Starting in 1994, the Wild Horse Inventory Reports began
not only reporting the number of wild horses identified in
specific areas, but also listing “designated management herd
minimum size[s]” (which appear to be analogous to what the
Service now terms “appropriate management levels”) for each
region. Those post-1994 reports continued to treat portions of
the Middle Section as part of the Wild Horse Territory by
designating a management herd minimum size for “Big Sage”
and “Boles [Meadow].” J.A. 857.
                                 18
    All subsequent Wild Horse Inventory Reports in the
record—for the years 1996, 1997, 1998, 2002, 2004, and
2010—also established “designated management herd
minimum sizes” for Big Sage and Boles Meadow. See, e.g.,
J.A. 851. 4 On top of that, the 2002 and 2010 Wild Horse
Inventory Reports listed “Avanzino”—also contained in the
Middle Section—as one of the regions surveyed.

     The pattern continues in the post-1994 Wild Horse
Inventory Reports. For instance, the 1998 Wild Horse
Territory Report recommended the relocation of wild horses
found in Boles Meadow. See J.A. 852 (1998 Wild Horse
Inventory Report: “Our plans in 1999 are to capture and
relocate 100 adult horses on the Emigrant Spring, Boles, Pine
Springs, [and] Surveyor’s Valley allotments.”) (emphasis
added). Other Wild Horse Inventory Reports recommended
that wild horses found outside the Wild Horse Territory be
relocated to Boles Meadow. See, e.g., J.A. 856 (1996 Wild
Horse Inventory Report: “Horse sign has been observed in the
Garden Tank and Lower Fletcher Creek areas. Th[ese] area[s]
[are] outside of the Wild Horse Territory and these horses
should be moved back over to the Timbered Ridge Area (Boles
[Meadow]).”) (emphasis added); J.A. 858 (same for 1994 Wild
Horse Inventory Report).

     Further, the 1997 Wild Horse Inventory Report
inventoried 289 wild horses in total, but noted that 40 of those
wild horses were found in the “Mtn. Dome Area,” which was
“outside * * * the * * * Devil’s   Garden      Wild     [H]orse
Territor[y].” J.A. 854. The remaining 249 wild horses—
including 17 wild horses in Big Sage and 36 wild horses in

    4
        If, as it appears, “designated management heard minimum
sizes” are synonymous with “appropriate management levels,” the
presence of those figures contradicts the Service’s claim that levels
were never set.
                                19
Boles Meadow—were deemed to be “within the territory.”
Compare J.A. 853 (inventory chart), with J.A. 854 (“Actual
horses counted were 249 head within the territory.”).
Similarly, in the 2002 Wild Horse Inventory Report, the
Service stated that the “[a]ctual horses counted were 500 head
within the wild horse territory.” J.A. 850 (emphasis added).
The inventory report shows that this 500 figure included 88
wild horses in Big Sage, 11 wild horses in Avanzino, and 64
wild horses in Boles Meadow. J.A. 850. Further, to reduce this
500 figure, the Service planned “to capture and adopt 160 adult
horses on the Emigrant Spring, Big Sage, and Avanzino
allotments.” J.A. 850. And “additional * * * horses [would]
need to be removed to be compliant with the Devil’s Garden
[Wild Horse Territory Plan].” J.A. 850 (emphasis added). 5

     The Service does not deny what the Inventory Reports say
or their treatment of wild horses in the Middle Section. Instead,
the Service says we should pay no mind to those reports
because those horses were “included solely for administrative
convenience.” Service’s Br. 36–37. For that proposition, the
Service points to a 2014 e-mail from a retired Service employee
who purportedly was responsible for creating the 1980s-era
Service map that first depicted the single, contiguous Wild
Horse Territory. The email states that the retired employee
“assigned Administrative [Appropriate Management Levels] to
the Grazing Allotments for ease of managing the existing
horses & the areas they were inhabiting.” J.A. 767 (emphasis
omitted). The email further states that “[t]he Administrative

    5
       The Wild Horse Inventory Reports also consistently surveyed,
and set appropriate management levels for, wild horses in the
Timbered Mountain Allotment. Most of the Timbered Mountain
Allotment is part of the original two-part Wild Horse Territory, but
part of it is in the Middle Section. The pasture within the Timbered
Mountain Allotment that is most heavily used by wild horses
includes area within the Middle Section.
                               20
map & [Appropriate Management Levels] were never intended
to change the [Wild Horse Territory] Plan,” and were only
made “for ease of managing the existing situation within the
[Wild Horse Territory].” J.A. 767.

     That email is at best a post hoc rationalization that was not
part of the record before the Service when it issued its 2013
decision, and thus cannot help the Service here. See District
Hosp. Partners v. Burwell, 786 F.3d 46, 55 (D.C. Cir. 2015)
(“To ensure that we review only those documents that were
before the agency, we do not allow parties to supplement the
record unless they can demonstrate unusual circumstances
justifying a departure from this general rule.”) (internal
quotation marks and citation omitted); American Wildlands v.
Kempthorne, 530 F.3d 991, 1002 (D.C. Cir. 2008).

     On top of that, the email explanation makes little sense.
What administrative ease or convenience is there for counting
in an inventory horses that should not count? Clearly the
Service understood the relevant boundaries of Big Sage and
Boles Meadow; otherwise, it could not have counted the wild
horses in those areas when preparing the Wild Horse Inventory
Reports. So if Big Sage and Boles Meadow were simply not
part of the Wild Horse Territory, all the Service had to do was
not include those wild horses in the Wild Horse Inventory
Reports, or note that Big Sage and Boles Meadow were not part
of the Wild Horse Territory. That was not hard to do; the
Service repeatedly did it for other wild horses found outside the
Wild Horse Territory. Convenience alone, then, does not
explain why the Service not only monitored and managed wild
horses in the Middle Section, but did so at the same time it
                                  21
refused to count all of the other horses that it deemed to be
outside of the Wild Horse Territory. 6

     In sum, the formal and published 1991 Forest Plan along
with at least two decades of official Wild Horse Inventory
Reports and the management activities they document together
demonstrate that for twenty years the Service officially treated
portions of the Middle Section as part of a single, contiguous
Devil’s Garden Wild Horse Territory. Given that longstanding
practice, it is unsurprising that the Service’s 2011 scoping letter
for the revision at issue here treated the Middle Section as part
of the Wild Horse Territory. While the agency tries to whistle
past that factual graveyard, the established pattern of agency
conduct and formalized positions cannot be evaded. The
Service’s failure even to acknowledge its past practice and
formal policies regarding the Middle Section, let alone to
explain its reversal of course in the 2013 decision, was arbitrary
and capricious. See, e.g., Encino Motorcars, 136 S. Ct. at
2126; West Deptford Energy, LLC v. FERC, 766 F.3d 10, 12
(D.C. Cir. 2014) (vacating agency action because, inter alia,
the agency “provided no reasoned explanation for how its
decision comports with * * * prior agency practice”); accord
Northwest Envtl. Def. Ctr. v. Bonneville Power Admin., 477
F.3d 668, 687, 690 (9th Cir. 2007) (invalidating an agency
action because the agency “depart[ed] from its long-standing

     6
       That is just the beginning of the e-mail’s credibility problems.
For instance, the email states that the retired employee “developed
the Administrative map lumping the [Wild Horse] [T]erritory into
one in the early 1990’s.” J.A. 767 (emphasis omitted). But the
parties agree that the map in question actually originated in the
1980s. The email further asserts that, “[i]n the early 1980’s Triangle
was acquired through a land exchange,” and “[b]ecause we had
horses in the areas,” he “lump[ed] them in with [the Wild Horse
Territory].” J.A. 767. But there is no dispute that the Service
acquired the Triangle lands in 1976.
                              22
practice” and “two-decade-old precedent without supplying a
reasoned analysis for its change of course”).

                               2

    The Service also trots out would-be legal impediments to
the Forest Plan’s inclusion of the Middle Section in the Wild
Horse Territory, insisting that it lacked the legal authority to
incorporate the Middle Section and failed to comply with the
procedural requirements for such a measure. But the Service’s
supposed failure to comply with all of the applicable laws for
amending the boundaries of a Wild Horse Territory a quarter
century ago does not mean that the expansion never happened.
A failure to comply with the requirements of the Wild Horses
Act or Administrative Procedure Act would not render the
change to the territory void from its inception.

     The Service’s assumption that a purported past mistake
would excuse the agency’s current missteps is wrong. In
administrative law, as elsewhere, two wrongs do not make a
right. In addition, regardless of whether the Service’s original
decision was lawful, the Service never grappled with the 1991
Forest Plan’s formal recognition of the unitary wild horse
territory, the inventory reports, or its actions monitoring and
regulating wild horses in the Middle Section for twenty years.
Accordingly, whatever the Service’s past transgressions, “we
cannot condone the ‘correction’ of one error by the
commitment of another.” Gray v. Mississippi, 481 U.S. 648,
663 (1987); see also Natural Res. Def. Council v. Thomas, 805
F.2d 410, 435 (D.C. Cir. 1986).

                               3

     Of course, the Service is free to change its policies going
forward if doing so is reasonable. But the agency first must at
least “display awareness that it is changing position” and
                               23
“show that there are good reasons for the new policy.” Fox
Television, 556 U.S. at 515. If instead “an agency glosses over
or swerves from prior precedents without discussion it may
cross the line from the tolerably terse to the intolerably mute.”
Greater Boston, 444 F.2d at 852, quoted in State Farm, 463
U.S. at 57; see also Encino Motorcars, 136 S. Ct. at 2125
(“Agencies are free to change their existing policies so long as
they provide a reasoned explanation for the change.”). 7

     Accordingly, if the Service wishes now to formally revert
to the 1975 territorial lines, it must acknowledge that it is
actually changing course and explain its reasons for doing so.
Whatever the cause of the initial incorporation of the Middle
Section into the Wild Horse Territory, that action morphed into
a multi-decade agency policy—formalized in part by the 1991
Forest Plan—that cannot be abandoned without some reasoned
explanation. The Service’s attempt to slam shut the barn door
after the horse already bolted is not sufficient.

                                B

     The Campaign next challenges the Service’s
determination that the removal of the Middle Section did not
constitute a significant amendment under the Forest
Management Act. The Forest Management Act provides that,
if “amendment” of a Forest Plan “would result in a significant
change in such [Forest Plan],” the Service must comply with
heightened procedural requirements including public
participation through a three-month study, public meetings,

    7
       See also Atchison, Topeka & Santa Fe Ry. Co. v. Wichita Bd.
of Trade, 412 U.S. 800, 808 (1973) (plurality opinion) (“Whatever
the ground for the departure from prior norms, however, it must be
clearly set forth [by the agency] so that the reviewing court may
understand the basis of the agency’s action and so may judge the
consistency of that action with the agency’s mandate.”).
                              24
and compliance with the Multiple Use Sustained Yield Act of
1960, 16 U.S.C. §§ 528–531. See 16 U.S.C. § 1604(d), (e),
(f)(4) (emphasis added).

    In its Final Environmental Assessment, the Service
concluded that amending the 1991 Forest Plan to redraw the
Wild Horse Territory boundary was a “non-significant” action,
and thus did not trigger the formal amendment process. The
Service instead complied with the Act’s less rigorous
procedural requirements for alterations to a Forest
Management Plan, 16 U.S.C. § 1604(d); see id. § 1604(f)(4).

    The Campaign argues that excision of the Middle Section
from the Wild Horse Territory made a “significant” change to
the Forest Plan itself, and thus triggered the Forest
Management Act’s more formal and elaborate review process.
That is not correct. While elimination of the Middle Section
worked an important and consequential change in the Wild
Horse Territory itself, the removal of those 23,000 acres had no
material impact on the 1991 Forest Plan governing 1.6 million
acres of the Modoc National Forest.

    In interpreting the Forest Management Act, the Forest
Service has explained that non-significant changes include
“[a]djustments of management area boundaries or management
prescriptions resulting from further on-site analysis when the
adjustments do not cause significant changes in the multiple-
use goals and objectives for long-term land and resource
management.” FOREST SERV., U.S. DEP’T OF AGRICULTURE,
FOREST SERVICE MANUAL § 1926.51 (2015). By contrast, to
constitute a “significant change” to a Forest Plan, the change
would have to “significantly alter the long-term relationship
between levels of multiple-use goods and services originally
projected,” “have an important effect on the entire [Forest
Plan],” or “affect land and resources throughout a large portion
                                  25
of the planning area during the planning period.”                   Id.
§ 1926.52. 8

     Viewed against the comprehensive scope and operation of
the 1991 Forest Plan, the Service’s reworking of the Wild
Horse Territory boundaries was not a significant change. The
1991 Forest Plan governs the entire Modoc National Forest. In
formulating the plan, the Service balanced a panoply of
considerations, including air quality, cultural resources,
biodiversity, fire management, mineral leasing, pest control,
range conditions, range management, livestock grazing,
recreational uses, riparian resources, soil and water quality,
timber clearance, and wildlife protection.

     Within the Forest Plan, wild horse management is just one
consideration, among a multitude of others, that fall within the
broader consideration of the Plan’s range-management sub-
component. Other factors that the Service considers in
developing a range management program are livestock
management and diversity of ecological conditions. For
instance, one objective of range management is “to produce
desired expressions of” “herbaceous, shrub, and forest
vegetation” through livestock grazing. J.A. 585. And even
within the subject of wild horse management, the physical
dimensions of the wild horse territory are just one element.

     In other words, wild horse management is a factor of a
factor of a factor that the Service considered when developing
the Modoc Forest Plan. In addition, the Service’s proposed
reduction of the Wild Horse Territory by approximately 23,000

     8
         The Forest Service Manual is a compendium of “legal
authorities, responsibilities, delegations, and general instruction[s],”
36 C.F.R. § 216.2(a), that “establish[es] the general framework for
the management and conduct of Forest Service programs,” id.
§ 216.2(c).
                                26
acres represents at most a 1.5% change in the status of an area
that will still remain within the federally managed Modoc
National Forest. Even then, the Service will continue to
superintend the remaining Wild Horse Territory, balancing
wild horse management against livestock management and
range biodiversity, and then balancing all of those range-
management considerations against such concerns as air, soil,
and water quality, and all of the other aspects of forest
management.

     When viewed in that context, the Service reasonably
concluded that its proposed modification of the Wild Horse
Territory’s boundaries would likely have at most a slight effect
on the “multiple-use” goals of the Forest Plan in the “long-
term,” and will not affect the “entire” Forest Plan or a “large
portion” of the planning area. 9

     Finally, because we hold that the boundary change was not
a “significant” change for purposes of the Forest Management
Act, we need not address the Campaign’s argument that NEPA
requires an Environmental Assessment for significant changes
under the Forest Management Act.

                                C

     The Campaign’s final argument arises under NEPA and
the APA. Specifically, the Campaign argues that the Service’s
failure to prepare an Environmental Impact Statement
addressing the boundary changes violated NEPA and was the
product of arbitrary and capricious decisionmaking. We agree.

    9
       The Campaign does not challenge the criteria set forth in the
Forest Service Manual for a “significant” change. Accordingly, we
need not determine what level of deference is properly afforded to
the Service’s interpretation of the Act. Cf. Baptist Mem’l Hosp.-
Golden Triangle v. Sebelius, 566 F.3d 226, 228 (D.C. Cir. 2009).
                               27
In finding no significant environmental impact that would
warrant an Environmental Impact Statement, the Service’s
NEPA analysis never came to grips with its departure from past
practice, and thus never analyzed the potential environmental
significance of its 2013 decision to contract the boundaries of
the Wild Horse Territory by approximately ten percent.

     Our “role in reviewing an agency’s decision not to prepare
an [Environmental Impact Statement] is a ‘limited’ one,
‘designed primarily to ensure that no arguably significant
consequences have been ignored.’” Myersville Citizens for a
Rural Community, Inc. v. FERC, 783 F.3d 1301, 1322 (D.C.
Cir. 2015) (quoting TOMAC v. Norton, 433 F.3d 852, 860
(D.C. Cir. 2006)). Our task in particular is to ensure that the
Service, in finding no significant impact, (i) “accurately
identified the relevant environmental concern,” (ii) took a
“hard look at the problem” in making its decision, (iii) has
made “a convincing case for its finding of no significant
impact,” and (iv) “has shown that even if there is an impact of
true significance, an [Environmental Impact Statement] is
unnecessary because changes and safeguards in the project
sufficiently reduce the impact to a minimum.” Sierra Club v.
Van Antwerp, 661 F.3d 1147, 1154 (D.C. Cir. 2011) (quoting
TOMAC, 433 F.3d at 861); see also Sierra Club v. Department
of Transp., 753 F.2d 120, 127 (D.C. Cir. 1985).

     The Service failed that task because its environmental
analysis did not “accurately identif[y] the relevant
environmental concern.” Van Antwerp, 661 F.3d at 1154
(emphases added; internal quotation marks and citation
omitted). Here, the relevant environmental concern was the
effect of the boundary modification on the wild horse
population in the Devil’s Garden area. The Service not only
failed to address that concern, it denied its very existence. The
Service insisted that the redrawn boundary lines would have
                               28
“no effect” on the ground because it was only “correct[ing] a
boundary established for administrative convenience.” J.A.
372. That is, the only sense in which the Service “identified”
the effect of the boundary modification on wild horses was by
insisting that there was “no effect” and that nothing had ever
really changed. That head-in-the-sand approach to past agency
practice is the antithesis of NEPA’s requirement that an
agency’s environmental analysis candidly confront the relevant
environmental concerns. 10

     More to the point, because the Service actually managed
wild horses within portions of the Middle Section for two
decades as though they were within the Wild Horse Territory,
the 2013 boundary change entailed far more than scratching out
a few lines in the 1991 Forest Plan. Yet the Service refused to
even consider the possibility of that broader, real-world impact.
Thus, while the Service “identifie[d]” some effects on wild
horses as an environmental concern, the Service did not
forthrightly and “accurately identif[y] the relevant
environmental concern”—the actual effects of the boundary
modification on wild horses in the Devil’s Garden area. Van
Antwerp, 661 F.3d at 1154 (emphases added; internal quotation
marks and citation omitted).

     Likewise, the Service’s Finding of No Significant Impact
not only failed to take a “hard look” at the consequences of the
boundary change, it averted its eyes altogether. Van Antwerp,
661 F.3d at 1154. As a result, the Service’s analysis entirely
omitted a “discussion of the relevant issues and opposing
viewpoints,” Myersville, 783 F.3d at 1325 (internal quotation

    10
         In its August 2013 Environmental Assessment, the Service
also referred to the inclusion of the Middle Section as an
“administrative error.” J.A. 264. Saying something was both done
for “administrative convenience” and because of “an administrative
error” is incoherent. The former is purposeful; the latter is not.
                                29
marks and citation omitted), because the Service refused even
to entertain the opposing premise that the boundary
modification did more than correct a few stray lines of text in
the 1991 Forest Plan. The Service’s decision thus failed to
“make a convincing case for its finding of no significant
impact.” Van Antwerp, 661 F.3d at 1154 (internal quotation
marks and citation omitted).

    For those reasons, the Service’s environmental analysis
and Finding of No Significant Impact were arbitrary and
capricious. 11

                             *****

     “Facts are stubborn things.” 12 But record facts are the grist
of reasoned agency decisionmaking. In this case, the Service
brushed aside critical facts about its past treatment of and

    11
          The Campaign, in a footnote, also lobs an accusation of
prejudgment and bad-faith at the Service, charging it with acting at
the behest of the Farm Bureau. The Campaign has not come close to
meeting the high bar necessary to prove such allegations. See, e.g.,
Air Transp. Ass’n of America, Inc. v. National Mediation Bd., 663
F.3d 476, 487–488 (D.C. Cir. 2011). Regardless, its footnoted
allegation is insufficient to preserve this argument on appeal. See
CTS Corp. v. EPA, 759 F.3d 52, 64 (D.C. Cir. 2014) (“A footnote is
no place to make a substantive legal argument on appeal; hiding an
argument there and then articulating it in only a conclusory fashion
results in forfeiture.”).
    12
         See Statement of John Adams Made When Defending the
Accused British Regulars at the Boston Massacre Trial, quoted in
WILLIAM GORDON, THE HISTORY OF THE RISE, PROGRESS, AND
ESTABLISHMENT, OF THE INDEPENDENCE OF THE UNITED STATES OF
AMERICA: INCLUDING AN ACCOUNT OF THE LATE WAR; AND OF
THE THIRTEEN COLONIES, FROM THEIR ORIGIN TO THAT PERIOD,
Vol. 1, p. 296 (1788).
                              30
official statements about the boundaries of the Devil’s Garden
Wild Horse Territory. As a result, the Service failed: (i) to
acknowledge and adequately explain its change in course
regarding the size of the Devil’s Garden Wild Horse Territory
and its management of wild horses within the Middle Section,
and (ii) to consider or to adequately analyze the environmental
consequences of those changes. Accordingly, we reverse the
district court’s grant of summary judgment in part and direct
the district court to remand to the Service for further
consideration.

                                                   So ordered.
