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The Navy then filed an emergency motion in the Court of Appeals requesting immediate vacatur of the District Court’s modified injunction. CEQ’s action, the Navy urged, eliminated the injunction’s legal foundation. In the alternative, the Navy sought a stay of two aspects of the injunction pending its appeal: the 2,200-yard mandatory shutdown zone and the power-down requirement in significant surface ducting conditions, see ante, at 7–8.
While targeting in its stay application only two of the six measures imposed by the District Court, the Navy explicitly reserved the right to challenge on appeal each of the six mitigation measures. Responding to the Navy’s emergency motion, the Court of Appeals remanded the matter to allow the District Court to determine in the first in-stance the effect of the intervening executive action. Pending its own consideration of the Navy’s motion, the District Court stayed the injunction, and the Navy con-ducted its sixth exercise.
On February 4, after briefing and oral argument, the District Court denied the Navy’s motion. The Navy appealed, reiterating its position that CEQ’s action eliminated all justification for the injunction. The Navy also argued that vacatur of the entire injunction was required irrespective of CEQ’s action, in part because the “conditions imposed, in particular the 2,200 yard mandatory shutdown zone and the six decibel (75%) power-down insignificant surface ducting conditions, severely degrade the Navy’s training.” Brief for Appellants in No. 08–55054 (CA9), p. 15. In the February 29 decision now under review, the Court of Appeals affirmed the District Court’s judgment. 518 F. 3d 658, 703 (2008). The Navy has continued training in the meantime and plans to complete its final exercise in December 2008.
As the procedural history indicates, the courts below determined that an EIS was required for the 14 exercises. The Navy does not challenge that decision in this Court. Instead, the Navy defends its failure to complete an EISb efore launching the exercises based upon CEQ’s “alternative arrangements”—arrangements the Navy sought and obtained in order to overcome the lower courts’ rulings. As explained below, the Navy’s actions undermined NEPA and took an extraordinary course.

II

NEPA “promotes its sweeping commitment” to environ-mental integrity “by focusing Government and public attention on the environmental effects of proposed agency action.” Marsh v. Oregon Natural Resources Council, 490
U. S. 360, 371 (1989). “By so focusing agency attention, NEPA ensures that the agency will not act on incomplete information, only to regret its decision after it is too late to correct.” Ibid.
The EIS is NEPA’s core requirement. Department of Transportation v. Public Citizen, 541 U. S. 752, 757 (2004). This Court has characterized the requirement as “action-forcing.” Andrus v. Sierra Club, 442 U. S. 347, 350 (1979) (internal quotation marks omitted). Environmental concerns must be “integrated into the very process of agency decision-making” and “interwoven into the fabric of agency planning.” Id., at 350–351. In addition to discuss-ing potential consequences, an EIS must describe potential mitigation measures and alternatives to the proposed course of action. See Robertson v. Methow Valley Citizens Council, 490 U. S. 332, 351–352 (1989) (citing 40 CFR §§1508.25(b), 1502.14(f), 1502.16(h), 1505.2(c) (1987)).The EIS requirement “ensures that important effects will not be overlooked or underestimated only to be discovered after resources have been committed or the die otherwise cast.” 490 U. S., at 349.
“Publication of an EIS . . . also serves a larger informational role.” Ibid. It demonstrates that an agency has indeed considered environmental concerns, and “perhaps more significantly, provides a springboard for public comment.” Ibid. At the same time, it affords other affected governmental bodies “notice of the expected consequences and the opportunity to plan and implement corrective measures in a timely manner.” Id., at 350.

In light of these objectives, the timing of an EIS is critical. CEQ regulations instruct agencies to “integrate the NEPA process with other planning at the earliest possible time to insure that planning and decisions reflect environmental values.” 40 CFR §1501.2 (1987). An EIS must be prepared “early enough so that it can serve practically as an important contribution to the decision-making process and will not be used to rationalize or justify decisions already made.” Andrus, 442 U. S., at 351–352, n. 3 (quoting 40 CFR §1502.5 (1979)).

The Navy’s publication of its EIS in this case, scheduled to occur after the 14 exercises are completed, defeats NEPA’s informational and participatory purposes. The Navy’s inverted timing, it bears emphasis, is the very reason why the District Court had to confront the question of mitigation measures at all. Had the Navy prepared a legally sufficient EIS before beginning the SOCAL exercises, NEPA would have functioned as its drafters in-tended: The EIS process and associated public input might have convinced the Navy voluntarily to adopt mitigation measures, but NEPA itself would not have impeded the Navy’s exercises. See Public Citizen, 541 U. S., at 756, 769, n. 2 (noting that NEPA does not mandate particular results, but rather establishes procedural requirements with a “focus on improving agency decision-making”).

The Navy had other options. Most importantly, it could have requested assistance from Congress. The Government has sometimes obtained congressional authorization to proceed with planned activities without fulfilling NEPA’s requirements. See, e.g., Floyd D. Spence National Defense Authorization Act for Fiscal Year 2001, Pub. L. 106–398, §317, 114 Stat. 1654A–57 (exempting the military from preparing a programmatic EIS for low-level flight training); 42 U. S. C. §10141(c) (exempting the Environmental Protection Agency from preparing an EIS for the development of criteria for handling spent nuclear fuel and high-level radioactive waste); 43 U. S. C. §1652(d) (exempting construction of the trans-Alaska oil pipeline from further NEPA compliance).
Rather than resorting to Congress, the Navy “sought relief from the Executive Branch.” Ante, at 8. On January10, 2008, the Navy asked CEQ, adviser to the President, to approve alternative arrangements for NEPA compliance pursuant to 40 CFR §1506.11 (1987). App. L to Pet. for Cert. 233a; see ante, at 8, n. 3. The next day, the Navy submitted supplementary material to CEQ, including the Navy’s EA and after-action reports, the District Court’s orders, and two analyses by the National Marine Fisheries Service (NMFS). App. L to Pet. for Cert. 237a–238a. Neither the Navy nor CEQ notified NRDC, and CEQ did not request or consider any of the materials underlying the District Court orders it addressed.
Four days later, on January 15, the Chairman of CEQ issued a letter to the Secretary of the Navy. Repeating the Navy’s submissions with little independent analysis, the letter stated that the District Court’s orders posed risks to the Navy’s training exercises. See id., at 238a (“You have explained that the training restrictions set forth in the . . . injunctive orders prevent the Navy from providing Strike Groups with adequate proficiency training and create a substantial risk of precluding certification of the Strike Groups as combat ready.”).

The letter continued: “Discussions between our staffs, your letter and sup-porting documents, and the classified declaration and briefings I have received, have clearly determined that the Navy cannot ensure the necessary training to certify strike groups for deployment under the terms of the injunctive orders. Based on the record support-ing your request . . . CEQ has concluded that the Navy must be able to conduct the [exercises] . . . in a time-frame that does not provide sufficient time to complete an EIS. Therefore, emergency circumstances are present for the nine exercises and alternative arrangements for compliance with NEPA under CEQ regulation 40 C.F.R. §1506.11 are warranted.” Id., at 240a.

The alternative arrangements CEQ set forth do not vindicate NEPA’s objectives. The arrangements provide for “public participation measures,” which require the Navy to provide notices of the alternative arrangements. Id., at 242a. The notices must “seek input on the process for reviewing post-exercise assessments” and “include an offer to meet jointly with Navy representatives . . . and CEQ to discuss the alternative arrangements.” Id., at 242a–243a. The alternative arrangements also describe the Navy’s existing research and mitigation efforts. Id., at 243a–247a.
CEQ’s hasty decision on a one-sided record is no substitute for the District Court’s considered judgment based on a two-sided record.2 More fundamentally, even an exemplary CEQ review could not have effected the short circuit the Navy sought. CEQ lacks authority to absolve an agency of its statutory duty to prepare an EIS. NEPA established CEQ to assist and advise the President on environmental policy, 42 U. S. C. §4342, and a 1977 Executive Order charged CEQ with issuing regulations to federal agencies for implementation of NEPA’s procedural provisions, Exec. Order No. 11991, 3 CFR 123 (1977 Comp.). This Court has recognized that CEQ’s regulations are entitled to “substantial deference,” Robertson, 490
U. S., at 355, and §1506.11 indicates that CEQ may play an important consultative role in emergency circum-stances, but we have never suggested that CEQ could eliminate the statute’s command. If the Navy sought to avoid its NEPA obligations, its remedy lay in the Legislative Branch. The Navy’s alternative course—rapid, self-serving resort to an office in the White House—is surely not what Congress had in mind when it instructed agencies to comply with NEPA “to the fullest extent possible.”42 U. S. C. §4332.3
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