The U.S. Navy conducts integrated training exercises for carrier and expeditionary strike groups off the Southern California coast. These exercises use mid-frequency active (MFA) sonar, which the Navy considers essential for detecting increasingly quiet diesel-electric submarines. The Natural Resources Defense Council (NRDC) and other environmental organizations sued the Navy in the Central District of California, alleging that the Navy violated the National Environmental Policy Act (NEPA) by proceeding with a series of exercises based on an Environmental Assessment and a Finding of No Significant Impact (FONSI), rather than preparing a full Environmental Impact Statement (EIS). Plaintiffs cited scientific evidence and prior incidents suggesting that MFA sonar can disturb, injure, or disorient marine mammals, including whales, potentially causing strandings and other harms. The district court found that plaintiffs were likely to succeed on their NEPA claim and issued a preliminary injunction imposing mitigation measures on the Navy's training, such as expanded shutdown zones and restrictions under certain oceanographic conditions. While litigation continued, the Council on Environmental Quality (CEQ) invoked its "emergency circumstances" regulation to authorize alternative NEPA arrangements, and the President issued an exemption under the Coastal Zone Management Act (CZMA), mooting one coastal exclusion condition. The Ninth Circuit largely upheld two core NEPA-based mitigation measures: (1) a sonar shutdown when marine mammals are detected within 2,200 yards, and (2) a shutdown during "surface ducting" conditions that may amplify sound travel. The Navy petitioned for Supreme Court review, arguing that the injunction impeded critical training and that the lower courts applied the wrong injunction standard and misweighed the equities and public interest.
Whether courts may grant a preliminary injunction based on a mere possibility of irreparable harm and, applying the proper standard, whether NEPA-based restrictions on Navy MFA sonar training were warranted in light of the balance of equities and the public interest in military readiness.
A plaintiff seeking a preliminary injunction must establish: (1) that he is likely to succeed on the merits; (2) that he is likely to suffer irreparable harm in the absence of preliminary relief; (3) that the balance of equities tips in his favor; and (4) that an injunction is in the public interest. A "possibility" of irreparable harm is insufficient. NEPA imposes procedural duties and does not mandate injunctive relief; courts must apply traditional equitable principles, including balancing competing public interests, before enjoining government action (see also Amoco Production Co. v. Village of Gambell). Particular deference is owed to the professional judgment of military authorities in assessing the public interest and national security implications.
Reversing the Ninth Circuit, the Supreme Court held that the "possibility of irreparable harm" standard is too lenient; irreparable harm must be likely. Assuming arguendo that plaintiffs could satisfy the first two factors, the balance of equities and the public interest strongly favored the Navy. Accordingly, the Court vacated the two challenged mitigation measures—the 2,200-yard shutdown zone and the shutdown during surface ducting conditions—imposed as preliminary injunctive relief.
1) Clarifying the injunction standard: The Court reaffirmed the traditional four-factor test, emphasizing that irreparable harm must be likely, not merely possible. The Ninth Circuit's contrary formulation lowers the threshold and risks routine injunctions without a sufficient showing. 2) Application to this case—first two factors: The Court did not resolve whether plaintiffs were likely to succeed on the merits of their NEPA claim or had shown likely irreparable harm under the corrected standard. Instead, it assumed in plaintiffs' favor on those points to focus on the third and fourth factors. 3) Balance of equities and public interest: The Court found that the district court and Ninth Circuit undervalued the Navy's compelling interest in realistic, integrated training to prepare strike groups for deployment and submarine threats. The Navy's evidence showed that limits like a 2,200-yard shutdown and a categorical shutdown during surface ducting would materially degrade training by eliminating or curtailing scenarios that sailors are likely to confront in theater, thereby creating risks to service members and national security. Against these harms, the environmental injuries identified—while not trivial and including potential disturbances and temporary or even permanent auditory effects to some marine mammals—were not shown to approach the gravity of the national security concerns in the limited time frame of the training exercises. 4) NEPA's procedural character: NEPA ensures informed decision-making, not specific environmental outcomes. Even assuming a NEPA violation, equitable relief is not automatic. Courts must weigh concrete harms on both sides. The lower courts erred by crafting sweeping interim measures without adequately accounting for the government's strong showing of public interest in combat readiness and by effectively treating a procedural claim as warranting substantive operational constraints. 5) Deference to military judgment: The Court underscored that the balance and public interest assessments must account for the special competence of military authorities regarding training needs and national security risks. While courts remain responsible for enforcing law, they should not lightly second-guess professional military determinations about operational requirements. Because the equities and public interest tipped strongly in the Navy's favor, the preliminary injunction could not stand, regardless of any assumed showing on likelihood of success or irreparable harm.
Winter is the leading modern authority on preliminary injunctions. It rejects the Ninth Circuit's lenient "possibility" standard and insists on a likelihood of irreparable harm, along with a genuine balancing of equities and public interest. The case has become a staple citation across civil, administrative, environmental, and constitutional litigation. For environmental law, Winter underscores that NEPA is procedural and does not create an entitlement to injunctive relief. Courts must tailor remedies and consider countervailing public interests, including national security and agency expertise. Post-Winter, plaintiffs must marshal concrete, persuasive evidence of likely irreparable injury and be prepared to address public interest factors head-on. The decision also influenced related cases (e.g., eBay v. MercExchange; Monsanto v. Geertson Seed Farms; Nken v. Holder) in emphasizing the four-factor framework and rejecting categorical presumptions.
Winter v. NRDC is a pivotal remedies case that reshaped preliminary injunction practice. By requiring likely irreparable harm and rigorous consideration of the balance of equities and public interest, the Court curtailed lower courts' tendency to issue interim environmental injunctions on thin showings. The decision also highlighted NEPA's procedural character and the need to respect competing public interests, particularly national security.