The Military history of the United States during World War II covers the involvement of the United States during World War II. The Empire of Japan declared war on the United States of America and the British Empire on 7 December 1941, immediately after the attack on Pearl Harbor on the same day. On 11 December 1941, Germany and Italy also declared war on the United States. Until that time, the United States had maintained neutrality, although it had, since March that same year, supplied the British with war materials through the Lend-Lease Act. The British then went on to supply a significant part of that aid to the Soviet Union and its Western Allies. Between the United States entry on 8 December 1941 and the end of the war in 1945, over 16 million Americans served in the United States military. Many others served with the Merchant Marine and paramilitary civilian units like the WASPs.