Before entering World War II, the U.S. Navy conducted an undeclared war with Nazi Germany's U-Boats. Here's What You Need to Know: A full-scale war between the United States and Germany loomed on the horizon. America was not at war, but American sailors were dying when American-owned ships were torpedoed by German submarines. In 1941, few Americans knew of the destroyers USS Niblack, USS Kearny, and USS Reuben James, but these and other American warships were fighting a grim naval war months before the Japanese attack on Pearl Harbor. American warships fought this undeclared war in the bitterly cold waters of the North Atlantic. They often struggled through brutal sub-zero temperatures and rapid, violent weather changes. Heavy storms were common; storms developed so quickly that it was often impossible to predict them. In the winter, ice sometimes almost a yard thick covered the convoy ships and threatened to capsize them. Ships sank and sailors died within minutes in the icy water.