Kantokuen

Kantokuen (Japanese: 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 favour 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. Initially defensive in nature, they envisioned an invasion by the Red Army into Chinese territory that would then be parried by a Japanese counterattack from Korea, with the decisive battlefield being southern Manchuria. After the Japanese invaded Manchuria in 1931, Japanese and Soviet troops found themselves facing one another along a border thousands of kilometers in length. To protect the puppet state of Manchukuo and to seize the initiative early against the Red Army, the IJA adopted a policy of halting any Soviet advance along the border and fighting the greater part of the war in Siberia, an "epoch-making change" in Japanese strategic thought. The transition from the strategic defensive to the strategic offensive would not be reversed until 1945, when Japan was facing disaster during the Pacific War. Moreover, Japanese plans progressively increased in scope from relatively small-scale operations to gigantic multistage campaigns, which envisioned seizing virtually the entire Soviet Far East as far as Lake Baikal.

Japanese-Soviet relations reached a low point by 1937, with an increasing sense of crisis on both sides. The Kwantung Army, Japan's occupation force in Manchuria, was openly hostile to the Soviets and appeared to be spoiling for a fight. This army, which expanded over time from a minor garrison command into a full-fledged army group, acted as a "self-contained, autonomous" entity almost entirely independent from the central government. The Kwantung Army's history was punctuated by a long record of insubordination and unilateral military aggression, which Tokyo often weakly accepted as a fait accompli. As the Kwantung Army's power increased, the Soviet–Japanese border conflicts worsened, culminating in the Kanchazu Island incident in which a Soviet river gunboat was sunk by Japanese shore batteries, killing 37 personnel. These episodes, together with reciprocal political and military subversion by both sides led both Soviet and Japanese figures to conclude that a future showdown was likely, even inevitable.

After the outbreak of the Second Sino-Japanese War in July 1937, however, Japanese options in Manchuria suddenly became very limited, greatly reducing their potential threat to Siberia. The Soviets were eager to capitalize on this by signing the Sino-Soviet Non-Aggression Pact that year, and began supplying the Chinese with weapons and equipment. On February 13, 1938, Pravda triumphantly declared:

...the Japanese Army, which possesses a strength of about 1,200,000 men, 2,000 planes, 1,800 tanks, and 4,500 heavy artillery pieces, committed about 1,000,000 troops and a greater part of its arms in China.

— Pravda