The Japanese attack on Pearl Harbor and the subsequent German declaration of war against the United States in December 1941 led to another phase of the Battle of the Atlantic. In Operation Drumbeat and subsequent operations until August 1942, a large number of Allied merchant ships were sunk by submarines off the American coast as the Americans had not prepared for submarine warfare, despite clear warnings (this was the so-called Second happy time for the German navy). The situation became so serious that military leaders feared for the whole Allied strategy.