The States of Michigan, Ohio, Pennsylvania, Minnesota, and Wisconsin (plaintiffs) sought a preliminary injunction requiring the United States Army Corps of Engineers (defendant) and the Metropolitan Water Reclamation District of Greater Chicago (defendant) to take measures with respect to the Chicago Area Waterway System (CAWS) in order to prevent two invasive species of carp from entering and destroying the ecosystem of the Great Lakes. The plaintiffs presented evidence that the carp were near the Great Lakes, consumed small organisms upon which the whole food chain relied, reproduced rapidly, and would quickly come to dominate any new ecosystem while destroying the old system. The defendants conceded that the intrusion of the carp would destroy the CAWS ecosystem. However, the defendants also presented evidence that the plaintiffs’ requested injunctive relief would cause considerable harm to commerce, recreation, and public safety by blocking boat traffic, as well as by increasing the risk of flooding. Further, the defendants argued that various state agencies were taking measures to prevent the carp invasion. The federal district court denied the preliminary injunction. The plaintiffs appealed.