The Soviet Union, having first cooperated with Germany in invading Poland whilst remaining neutral in the Allied-Axis conflict, joined the Allies in late June 1941 after being invaded by Germany. The United States provided war materiel to the Allies beginning in March 1941 and officially joined in December 1941 after the Japanese attack on Pearl Harbor. China had already been in a prolonged war with Japan since the Marco Polo Bridge Incident of 1937, but officially joined the Allies during December 1941.