Background and buildup
The Russo-Japanese War of the early 20th century resulted in a Japanese victory and the Treaty of Portsmouth by which, in conjunction with other later events including the Mukden Incident and Japanese invasion of Manchuria in September 1931, Japan eventually gained control of Korea, Manchuria and Southern Sakhalin. In the late 1930s there were a number of Soviet-Japanese border incidents, the most significant being the Battle of Lake Khasan (Changkufeng Incident, July-August 1938) and the Battle of Khalkhin Gol (Nomonhan Incident, May-September 1939), which led to the Soviet–Japanese Neutrality Pact[7][8] of April 1941. The Neutrality Pact freed up forces from the border incidents and enabled the Soviets to concentrate on their war with Germany, and the Japanese to concentrate on their southern expansion into Asia and the Pacific Ocean.
However, with success at Stalingrad, the Soviet attitude to Japan changed, both publicly, with Stalin making speeches denouncing Japan, and “privately”, with the Soviets building up forces and supplies in the Far East. At the Tehran Conference (November 1943), amongst other things, Churchill, Roosevelt and Stalin agreed that the Soviet Union would enter the war against Japan once Nazi Germany was defeated. The Soviet buildup in the Far East continued, and by early 1945 it had become apparent to the Japanese that the Soviets were preparing to invade Manchuria; in addition to their problems in the Pacific, the Japanese realised they needed to determine when and where such an invasion would occur.
At the Yalta Conference (February 1945), amongst other things, Stalin secured from Roosevelt the promise of Stalin’s Far Eastern territorial desires, in return agreeing to enter the Pacific war within two or three months of the defeat of Germany. By the middle of March 1945, things were not going well in the Pacific for the Japanese, and they withdrew their elite troops from Manchuria to support actions in the Pacific. Meanwhile the Soviets continued their Far Eastern buildup. The Soviets had decided that they did not wish to renew the Neutrality Pact. The terms of the Neutrality Pact required that 12 months before its expiry, the Soviets must advise the Japanese of this, so on 5 April 1945 they informed the Japanese that they did not wish to renew the treaty.[9] This caused the Japanese considerable concern,[10][11] but the Soviets went to great efforts to assure the Japanese that the treaty would still be in force for another twelve months, and that the Japanese had nothing to worry about.[12]
On 9 May 1945 (Moscow time), Germany surrendered, meaning that if the Soviets were to honour the Yalta agreement, they would need to enter war with Japan by 9 August 1945. The situation continued to deteriorate for the Japanese, and they were now the only axis power left in the war. They were keen to keep at peace with the Soviets and extend the Neutrality Pact,[12] and they were also keen to achieve an end to the war. Continuously since Yalta they had repeatedly approached, or tried to approach, the Soviets in order to extend the neutrality pact, and to enlist the Soviets in negotiating peace with the allies. The Soviets did nothing to discourage these Japanese hopes, and drew the process out as long as possible (whilst continuing to prepare their invasion forces.)[12] One of the roles of the Cabinet of Admiral Baron Suzuki, which took office in April 1945, was to try to secure any peace terms short of unconditional surrender.[13] In late June they approached the Soviets, (the Neutrality Pact was still in place), inviting them to negotiate peace with the allies in support of Japan, providing them with specific proposals and in return they offered the Soviets very attractive territorial concessions. Stalin expressed interest, and the Japanese awaited the Soviet response. The Soviets continued to avoid providing a response. The Potsdam Conference was held from 16 July to 2 August 1945. On 24 July the Soviet Union recalled all embassy staff and families from Japan. On 26 July the conference produced the Potsdam Declaration whereby Churchill, Truman and Chiang Kai-shek (the Soviet Union was not at war with Japan) demanded the unconditional surrender of Japan. The Japanese continued to wait for the Soviet response, and avoided responding to the declaration.[12]
The Japanese had been monitoring Trans-Siberian Railway traffic and Soviet activity to the east of Manchuria and in conjunction with the Soviet delaying tactics, this suggested to them that the Soviets would not be ready to invade east Manchuria before the end of August. They did not, however, have any real idea, and no confirming evidence, as to when or where any invasion would occur.[14]
The Japanese were caught completely by surprise when the Soviets declared war an hour before midnight on 8 August 1945, and invaded simultaneously on three fronts just after midnight on 9 August.