Hill and other Tennessee residents (plaintiffs) filed suit in federal district court against the Tennessee Valley Authority (TVA) (defendant), a wholly-owned public corporation of the United States, seeking to enjoin construction of a nearly-completed dam and reservoir on the Little Tennessee River. Plaintiffs alleged the project failed to conform to the requirements of the National Environmental Policy Act (NEPA). The district court granted plaintiffs’ request and halted the project pending the filing of an Environmental Impact Statement. Several months later, a University of Tennessee scientist discovered a previously-unknown species of perch, called the snail darter, living in the waters affected by the project. Pursuant to the Endangered Species Act (ESA), the Secretary of the Interior found that once the dam and reservoir were operational, it would result in the “total destruction of the snail darter’s habitat.” The Secretary listed the snail darter on the endangered species list and declared that, under § 7 of the ESA, all federal agencies were required to protect the snail darter’s critical habitat area. Plaintiffs then filed suit against the TVA to enjoin the final completion of the dam and reservoir on the ground that it could result in the extinction of the snail darter. The district court denied plaintiffs’ request for injunction and dismissed the complaint. Plaintiffs appealed. The court of appeals reversed and remanded the case with instructions that a permanent injunction be issued. The U.S. Supreme Court granted certiorari to review.