Winter v. Natural Resources Defense Council, Inc.

The Navy obtained an exemption to use mid-frequency active sonar during training exercises off Southern California, having prepared only an environmental assessment rather than a full environmental impact statement. Environmental groups (plaintiffs) sued, arguing the sonar harmed marine mammals and that a full EIS was required; the district court found merely a 'possibility' of irreparable harm sufficient and granted a preliminary injunction restricting sonar use, which the court of appeals affirmed.

Whether, in awarding injunctive relief, a court must balance the competing claims of injury and consider the effect the requested relief will have on each party.