Kantokuen (関特演, from 関東軍特種演習, Kantōgun Tokushu Enshū, "Kwantung Army Special Maneuvers") was an operational plan created by the General Staff of the Imperial Japanese Army for an invasion and occupation of the Russian Far East, capitalizing on the outbreak of the Soviet-German War in June 1941. Involving seven Japanese armies and a major portion of the empire's naval and air forces, it would have been the largest combined arms operation in Japanese history up to that point, and one of the largest of all time.
The plan was approved in part by Emperor Hirohito on July 7 and involved a three-step readiness phase followed by a three-phase offensive to isolate and destroy the Soviet defenders within six months. After growing conflict with simultaneous preparations for an offensive in Southeast Asia, together with the demands of the Second Sino-Japanese War and dimming prospects for a swift German victory in Europe, Kantokuen fell out of favor at Imperial General Headquarters and was eventually abandoned after increased economic sanctions by the United States and its allies.
Nevertheless, the presence of large Japanese forces in Manchuria forced the Soviets, who had long anticipated an attack from that direction, to retain considerable military resources in Siberia throughout World War II.
The roots of anti-Soviet sentiment in Imperial Japan existed before the foundation of the Soviet Union itself. Eager to limit tsarist influence in East Asia after the Russo-Japanese War (1904–1905) and then to contain the spread of Bolshevism during the Russian Civil War, the Japanese deployed some 70,000 troops into Siberia from 1918 to 1922 as part of their intervention on the side of the White Movement, occupying Vladivostok and other key points east of Lake Baikal. After the international withdrawal from Russian territory and the establishment of the Soviet regime, the Imperial Japanese Army, mindful of the Soviets' potential as a military power and in keeping with the convention of Russia as a traditional enemy, made contingency plans for a future war.