The United States Navy (defendant) conducted military testing operations on the Vieques Island off the coast of Puerto Rico (plaintiff). These operations included discharging what amounted to pollutants into the waters off the island. Congress passed the Federal Water Pollution Control Act, which required a National Pollutant Discharge Elimination System (NPDES) permit for the discharge of any pollutant into navigable waters. The act’s purpose was to restore and maintain the integrity of the country’s waters. The act provided for phased-in compliance over a series of years. Puerto Rico sued the Navy based on the act, seeking an order enjoining the Navy from any further operations on or near the island. The district court ordered the Navy to apply for an NPDES permit. The district court denied Puerto Rico’s requested injunction. The United States Court of Appeals for the First Circuit vacated the order and remanded the case for entry of Puerto Rico’s requested injunction. The United States Supreme Court granted certiorari.