The defeat of Japan's Kwantung Army helped bring about the Japanese surrender and the end of World War II.

[5][13][14][15][16][17] At the Tehran Conference in November 1943, Joseph Stalin agreed that the Soviet Union would enter the war against Japan once Germany was defeated.

British participants included Winston Churchill, Anthony Eden, Field Marshal Sir Alan Brooke, and General Hastings Ismay.

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 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.

[20][21] At one minute past midnight Trans-Baikal time on 9 August 1945, the Soviets commenced their invasion simultaneously on three fronts to the east, west and north of Manchuria.

[citation needed] 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.

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.

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.

[29] 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.

[29] 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.

[17] 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.

[5] The Transbaikal Front, under Marshal Rodion Malinovsky, was to form the western half of the Soviet pincer movement and to attack across the Inner Mongolian desert and over the Greater Khingan mountains.

[32] These forces had the objective to secure Mukden (now Shenyang), then meet troops of the 1st Far East Front at the Changchun area in south-central Manchuria[5] and so end the double envelopment.

[5] Its final objective was to link up with forces of the Trans-Baikal Front at Changchun and Jilin thus closing the double envelopment movement.

The Imperial Japanese Navy did not contribute surface forces to the defense of Manchuria, the occupation of which it had always opposed on strategic grounds.

Additionally, by the time of the invasion, the few remnants of its fleet were stationed in defense of the Japanese home islands in anticipation of a possible invasion by Western Allied forces, however, IJN Naval Infantry units from 12th Air Fleet saw extensive action during South Sakhalin and Kuil Islands campaign.

[34] Most of its heavy equipment and all of its best troops had been transferred to the Pacific Front over the previous three years, with second-rate units raised to replace them.

As a result, it had essentially been reduced to a light infantry counterinsurgency force with limited mobility or ability to fight a conventional land war against a co-ordinated enemy.

[35] Compounding the problem, the Japanese military made many wrong assumptions and major mistakes, the two most significant the following: The withdrawal of the Kwantung Army's elite forces for redeployment into the Pacific Theatre made new operational plans for the defence of Manchuria against a seemingly-inevitable Soviet attack prepared by the Japanese in the summer of 1945.

That confused the Japanese military analysis of Soviet logistics, and the defenders were caught by surprise in unfortified positions.

The Kwantung Army had a formidable reputation as fierce and relentless fighters, and even though weak and unprepared, they put up strong resistance in the town of Hailar, which tied down some of the Soviet forces.

In the "Sixty Years after Hiroshima" issue of The Weekly Standard, the American historian Richard B. Frank points out that there are a number of schools of thought with varying opinions of what caused the Japanese to surrender.

He argues that Japan's leaders were impacted more by the swift and devastating Soviet victories on the mainland in the week after Stalin's 8 August declaration of war because the Japanese strategy to protect the home islands was designed to fend off an Allied invasion from the south and left virtually no spare troops to counter a Soviet threat from the north.

Ward Hayes Wilson, however, has argued extensively that the Soviet declaration of war was the sole cause of Japan's surrender.

He cites the June 1945 meeting of the Supreme Council at which they concluded that maintaining Soviet neutrality would "determine the fate of the Empire," and the general lack of regard for the importance of city bombing in Japan's ruling circles.

[56][57][58] The Soviet invasion and occupation of the defunct Manchukuo marked the start of a traumatic period for the more than one million residents of the puppet state who were of Japanese descent.

With Soviet support for the spread of communism,[62] Manchuria provided the main base of operations for Mao Zedong's forces, who proved victorious in the following four years of the Chinese Civil War.

[30] After the Communist victory in the Chinese Civil War and the establishment of the People's Republic of China in 1949, the USSR began providing economic assistance to the new government, the bulk of which went into rebuilding Manchuria's industrial base.

[63][full citation needed] As agreed at Yalta, the Soviet Union had intervened in the war with Japan within three months of the German surrender and so was therefore entitled to annex the territories of South Sakhalin, which Russia had lost to Japan in aftermath of the Russo-Japanese War, and the Kuril Islands and also to preeminent interests over Port Arthur and Dalian, with its strategic rail connections, via the China Changchun Railway, a company owned jointly by China and the Soviet Union that operated all railways of the former Manchukuo.