Stalin declared war on Japan at the request of Roosevelt and Churchill at the Yalta Conference. The war was fought in Manchuria and the islands. The USSR defeated the imperial Kwantung army numbering 700 thousand people. Nevertheless, not a single territory was annexed to the USSR. The USSR returned half of the island of Sakhalin, annexed by Japan in 1905 and several islands. Now the dispute is about these islands..
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The Soviet treaty of neutrality with Japan, like its earlier pact with Nazi Germany in August 1939, had never been thought of by either party as anything more than a mutually convenient but strictly temporary arrangement. For the Soviets, it allowed their forces to concentrate against Adolf Hitler’s 1941 invasion; at the same time, it freed Japan, after a very unsuccessful test of arms against the Soviets in Khalkin Gol, to focus its efforts on conquering China and fighting the Western powers in the Pacific.
During the Potsdam Conference in May 1945, however, Josef Stalin pledged to commit his forces to the Allied cause in the Pacific three months after Germany surrendered. After secretly transporting much of its army across the vast length of Siberia, the Soviet Union broke relations with Japan, declared war and plunged into Manchuria on August 9—right on schedule. By September some Soviet forces had made some landings in the Kurile Islands, but their relative inexperience in amphibious warfare, combined with the usual spirited Japanese resistance, limited their progress before all armed forces stood down.
Behind his promise to the Allies, of course, Stalin had hoped to make some inroads into the Far East and got what he wanted—among other things, payback by retaking Port Arthur and the establishment of a pro-Soviet regime in North Korea, though the spread of communism into China did not turn out quite the way he would have preferred.
After seeing what the soviets were doing in Europe, the US was in a hurry to capitulate Japan into surrender thus negating any need for soviet intervention.
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