As Axis forces began invading northern Europe and the Balkans, the Allies added Netherlands, Belgium, Norway, Greece, and Yugoslavia. The Soviet Union, which had a nonaggression pact with Germany and joined in its invasion of Poland, became an Allied member in June 1941 after being attacked by the Axis. The United States, while providing materiel and financial support to the Allies from September 1940, remained formally neutral until the Japanese bombing of Pearl Harbor in December 1941, after which it declared war and officially joined the Allies. China was already at war with Japan since 1937 but formally joined the Allies in December 1941.