The Soviet–Japanese War was a campaign of the Second World War that began with the Soviet invasion of Manchuria following the Soviet declaration of war against Japan on 8 August 1945. The Soviet Union and Mongolian People's Republic toppled the Japanese puppet states of Manchukuo in Manchuria and Mengjiang in Inner Mongolia, as well as northern Korea, Karafuto on the island of Sakhalin, and the Kuril Islands. The defeat of Japan's Kwantung Army helped bring about the Japanese surrender and the end of World War II. The Soviet entry into the war showed that the Soviet Union would not act as a third party in negotiating a conditional end to hostilities, and thus was a factor in the Japanese government's decision to surrender unconditionally.Richard B. Frank, Downfall: The End of the Imperial Japanese Empire, Penguin, 2001 . (Extracts on-line)

Summary At the Tehran Conference in November 1943, Joseph Stalin agreed that the Soviet Union would enter the war against Japan once Germany was defeated.

An Anglo-American delegation visited Moscow in October 1944 to discuss the Soviet Union joining the war against Japan. The meeting with Stalin on 15 October discussed the Soviet Union's participation, and Stalin said that the Soviet offensive could need American material assistance because of the limited freight capacity of the Trans-Siberian Railway; see Pacific Route. British participants included Winston Churchill, Anthony Eden, Field Marshal Sir Alan Brooke, and General Hastings Ismay. The American representative was W. Averell Harriman.

At the Yalta Conference in February 1945, Stalin agreed to Allied pleas to enter World War II in the Pacific Theater within three months of the end of the war in Europe.

On 26 July, the US, the UK, and China made the Potsdam Declaration, an ultimatum calling for the Japanese surrender that if ignored would lead to their "prompt and utter destruction".

The commencement of the invasion fell between the US atomic bombings of Hiroshima on 6 August and Nagasaki on 9 August. Although Stalin had been told virtually nothing of the US and UK's atomic bomb program by Allied governments, the date of the invasion was foreshadowed by the Yalta Agreement, the date of the German surrender (on 9 May 1945, implying a deadline for Stalin to enter the war against Japan by 9 August 1945), and the fact that, on 3 August, Marshal Aleksandr Vasilevsky reported to Stalin that, if necessary, he could attack on the morning of 5 August. The timing was well-planned and enabled the Soviet Union to enter the Pacific Theater on the side of the Allies, as previously agreed, before the war's end.Holloway, David. Stalin and the bomb: the Soviet Union and atomic energy, 1939–1956. Yale University Press, 1996. (p. 127–129) The proposed Soviet invasion of Hokkaido was originally planned to be part of the territory taken.Archive, Wilson Center Digital. Wilson Center Digital Archive, digitalarchive.wilsoncenter.org/document/122335. http://digitalarchive.wilsoncenter.org/document/122335

At 5 p.m. Moscow time (11 p.m. Trans-Baikal time (UTC+9)) on 8 August 1945, Soviet foreign minister Vyacheslav Molotov informed Japanese ambassador Naotake Satō in the Kremlin that the Soviet Union had declared war on Japan, and that from 9 August the Soviet Government would consider itself to be at war with Japan.Soviet Declaration of War on Japan, 8 August 1945. (Avalon Project at Yale University) At one minute past midnight Trans-Baikal time on 9 August 1945, or just over an hour after the declaration of war, the Soviets commenced their invasion simultaneously on three fronts to the east, west and north of Manchuria. The operation was subdivided into smaller operational and tactical parts: Khingan–Mukden Offensive Operation (9 August 1945 – 2 September 1945) Harbin–Kirin Offensive Operation (9 August 1945 – 2 September 1945) Battle of Mutanchiang Siege of Hutou Fortress Sungari Offensive Operation (9 August 1945 – 2 September 1945) and subsequently South Sakhalin Operation (11 August 1945 – 25 August 1945) Soviet assault on Maoka (19 August 1945 – 22 August 1945) Chongjin Landing Operation (13 August 1945 – 16 August 1945) Kuril Landing Operation (18 August 1945 – 1 September 1945) Battle of Shumshu

Though the battle extended beyond the traditional lands of the Manchu people, the coordinated and integrated invasions of Japan's northern territories has also been called the Battle of Manchuria.Maurer, Herrymon, Collision of East and West, Henry Regnery Company, Chicago, 1951, p.238. Since 1983, the operation has sometimes been called Operation August Storm, after United States Army historian Lieutenant Colonel David Glantz used this title for a paper on the subject. It has also been referred to by its Soviet name, the Manchurian Strategic Offensive Operation, but this name refers more to the Soviet invasion of Manchuria than to the whole war.

This offensive should not be confused with the Soviet–Japanese border conflicts (particularly the Battle of Khalkhin Gol of May–September 1939), that ended in Japan's defeat in 1939, and led to the Soviet–Japanese Neutrality Pact.

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 South Sakhalin. In the late 1930s 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 PactSoviet-Japanese Neutrality Pact, 13 April 1941. (Avalon Project at Yale University)Declaration Regarding Mongolia, 13 April 1941. (Avalon Project at Yale University) 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.

With success at the Battle of Stalingrad and the eventual defeat of Germany becoming increasingly certain, the Soviet attitude to Japan changed, both publicly, with Stalin making speeches denouncing Japan, and covertly, with the Soviets building up forces and supplies in the Far East. At the Tehran Conference (November 1943), Stalin, Churchill, and Roosevelt agreed that the Soviet Union would enter the war against Japan once Germany was defeated. Stalin faced a dilemma since he wanted to avoid a two-front war at almost any cost but also wanted to extract gains in the Far East as well as Europe. The only way that Stalin could make Far Eastern gains without a two-front war would be for Germany to surrender before Japan.

The Soviet–Japanese Neutrality Pact caused the Soviets to make it policy to intern Allied aircrews who landed in Soviet territory after operations against Japan, but airmen held in the Soviet Union under such circumstances were usually allowed to "escape" after some period of time. Nevertheless, even before the defeat of Germany, the Soviet buildup in the Far East had steadily accelerated. By early 1945, it had become apparent to the Japanese that the Soviets were preparing to invade Manchuria, but they were unlikely to attack prior to Germany's defeat. In addition to their problems in the Pacific, the Japanese realised that they needed to determine when and where a Soviet invasion would occur.

At the Yalta Conference (February 1945), Stalin secured from Roosevelt the promise of Stalin's Far Eastern territorial desires in return for agreeing to enter the Pacific War within two or three months of the defeat of Germany. By mid-March 1945, things were not going well in the Pacific for the Japanese, who 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 Neutrality Pact required that twelve months before its expiry, the Soviets must advise the Japanese and so on 5 April 1945, they informed the Japanese that they did not wish to renew the treaty.Soviet Denunciation of the Pact with Japan, 5 April 1945. (Avalon Project at Yale University) That caused the Japanese considerable concern,"So sorry, Mr Sato", April 1945, Time magazine.Russia and Japan , declassified CIA report from April 1945. 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.

On 9 May 1945 (Moscow Time), Germany surrendered and so 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, now the only Axis power left in the war. They were keen to remain at peace with the Soviets and extend the Neutrality PactBoris Nikolaevich Slavinskiĭ, The Japanese-Soviet Neutrality Pact: A Diplomatic History 1941–1945, Translated by Geoffrey Jukes, 2004, Routledge. (Extracts online) and also wanted to achieve an end to the war. Since Yalta, they had repeatedly approached or tried to approach the Soviets to extend the Neutrality Pact and to enlist the Soviets in negotiating peace with the Allies. The Soviets did nothing to discourage the Japanese hopes and drew the process out as long as possible but continued to prepare their invasion forces. 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.Jones, F. C. "Manchuria since 1931", 1949, Royal Institute of International Affairs, London. p.221 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 (in the Soviet-occupied German city of Potsdam) 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, Harry S. Truman and Chiang Kai-shek (the Soviet Union was not officially 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.

The Japanese had been monitoring Trans-Siberian Railway traffic and Soviet activity to the east of Manchuria and the Soviet delaying tactics, which suggested to them that the Soviets would not be ready to invade east Manchuria before the end of August. They did not have any real idea and no confirming evidence as to when or where any invasion would occur. They had estimated that an attack was not likely in August 1945 or before spring 1946, but Stavka had planned for a mid-August 1945 offensive and had concealed the buildup of a force of 90 divisions. Many had crossed Siberia in their vehicles to avoid straining the rail link.

Combatant forces

Soviets