The Sierra Club, an environmental interest group, and several businesses (plaintiffs) filed suit in federal court against the U.S. Army Corps of Engineers (Corps), the Federal Highway Administration (FHWA), and others (defendants), to halt construction of “Westway,” a super-highway running along the Hudson River in Manhattan, New York. Prior to issuing the required permits, the Corps prepared a Draft Environmental Impact Statement (DEIS) pursuant to the National Environmental Policy Act (NEPA), that recommended the highway be built atop landfill discharged into the River. A Final Environmental Impact Statement (FEIS) noted the area to be filled in was a “biological wasteland.” The Environmental Protection Agency, National Marine Fisheries Service, and the U.S. Fish & Wildlife Service objected to the EIS and recommended against issuance of any permits because the Corps had relied on outdated data to reach the conclusion that there would be no adverse impact to fish habitats in the River. The three agencies argued that a newer study showed rich populations of fish existed in area. Nevertheless, the Corps refused to prepare a supplemental EIS using the new data. The district court enjoined construction of the highway until a supplemental EIS could be prepared. Defendants appealed.