In Asia, German successes encouraged Japan to call for oil supplies from the Dutch East Indies .  Many Western countries reacted to the occupation of French Indochina by banning oil trading with Japan.  Japan planned to take over European colonies in Asia to create a great defensive area in the Pacific so that it could get more resources.  But before any future invasion, it first had to destroy the American Pacific Fleet in the Pacific Ocean .  On 7 December 1941, it attacked Pearl Harbor as well as many harbors in several South East Asian countries.  This event led the United States, United Kingdom, Australia, Western Allies and China to declare war on Japan, while the Soviet Union remained neutral.  Most of the Axis nations reacted by declaring war on the United States.
The Soviet Union offered support to the partisans in many Wehrmacht -occupied countries in Central Europe , notably those in Slovakia, Poland and the Kingdom of Yugoslavia . In addition, the Polish Armed Forces in the East , particularly the First and Second Polish armies, were armed and trained, and would eventually fight alongside the Red Army . The Free French forces also contributed to the Red Army by the formation of the GC3 ( Groupe de Chasse 3 or 3rd Fighter Group) unit to fulfill the commitment of Charles de Gaulle , leader of the Free French, who thought that it was important for French servicemen to serve on all fronts. A strategic air offensive by the United States Army Air Force and Royal Air Force played a significant part in reducing German industry and tying up German air force and air defense resources, with some bombings, such as the bombing of the eastern German city of Dresden , being done to facilitate specific Soviet operational goals. In addition to Germany, hundreds of thousands of tons of bombs were dropped on their eastern allies of Romania and Hungary , primarily in an attempt to cripple Romanian oil production . British and Commonwealth forces also contributed directly to the fighting on the Eastern Front through their service in the Arctic convoys and training Red Air Force pilots , as well as in the provision of early material and intelligence support. The later massive material support of the Lend-Lease agreement by the United States and Canada played a significant part particularly in the logistics of the war. Among other goods, Lend-Lease supplied: