The US armed forces were responsible for many tactical innovations during the years 1941-45, but in no field was US mastery more complete than amphibious warfare. The US armed forces pioneered amphibious warfare in the Pacific and later in the Mediterranean but it was against Fortress Europe that the most famous and the most successful amphibious assault took place. On D-Day the Western Allies invaded German-held Normandy in a pre-dawn amphibious assault by American, British and Canadian forces.