The United States declared war on Japan following the attack of December 7, 1941 on Pearl Harbor. The two nations revolutionized naval warfare in the course of the next four years; several of the most important sea battles were fought without either fleet coming within sight of the other. Most of the fleet carriers were built according to prewar designs, but the demand for air protection was so intense that two new classes were developed: light carriers (designated CVL), built on modified cruiser hulls, and escort carriers (CVE), whose main function was to protect Atlantic convoys from German U-boats.