The Navy was concerned with new diesel-electric submarines that are virtually silent and undetectable without intense training. Their weapon against such a threat is active sonar. Of concern to Ps were the Navy’s use of “mid-frequency active” (MFA) sonar, which transmits sound waves at frequencies between 1 kHz and 10 kHz. In the operating area, there are at least 37 species of marine mammals, including dolphins, whales, and sea lions. Ps contend that MFA active sonar training will harm those animals or disrupt their behavioral patterns. D emphasizes that it has used MFA sonar during training exercises for 40 years, without a single documented sonar-related injury to any marine mammal. Ps contend that it causes permanent hearing loss, decompression sickness, and major behavioral disruptions. Ps contend that several mass strandings of marine mammals (outside of SOCAL, the operating area) have been “associated” with the use of active sonar. Pursuant to NEPA D issued an EA concluding that SOCAL training exercises would not have a significant impact on the environment. Ps sued the D, seeking declaratory and injunctive relief on the grounds that the SOCAL training exercises violated NEPA. The District Court granted Ps' motion for a preliminary injunction and prohibited the Navy from using MFA sonar during its remaining training exercises. The Ninth Circuit stayed the injunction pending appeal. It concluded that a blanket injunction prohibiting the D from using MFA sonar in SOCAL was overbroad, and remanded the case to the District Court “to narrow its injunction. The District Court entered a new preliminary injunction allowing the Navy to use MFA sonar (1) imposing a 12-mile “exclusion zone” from the coastline; (2) using lookouts to conduct additional monitoring for marine mammals; (3) restricting the use of “helicopter-dipping” sonar; (4) limiting the use of MFA sonar in geographic “choke points”; (5) shutting down MFA sonar when a marine mammal is spotted within 2,200 yards of a vessel; and (6) powering down MFA sonar by 6 dB during significant surface ducting conditions, in which sound travels further than it otherwise would due to temperature differences in adjacent layers of water. D sought relief from the Executive Branch. The President, pursuant to 16 U. S. C. §1456(c)(1)(B), granted an exemption from the CZMA. The Council on Environmental Quality (CEQ) authorized D to implement “alternative arrangements” to NEPA compliance in light of “emergency circumstances.” D then moved to vacate the District Court’s injunction. The District Court refused to do so, and the Court of Appeals affirmed. The Court of Appeals determined that Ps had carried their burden of establishing a “possibility” of irreparable injury. The Supreme Court granted certiorari.