When discussing the Pacific campaign, I find most sources focus on the USMC, US Navy (especially carrier operations), followed occasionally by the disastrous British loss of Singapore, and finally by ANZAC cooperation with US forces.
However, Russia and Japan had an historical enmity including armed clashes in the run-up to the war. Were Soviet forces involved in the campaign against the Japanese Empire, and if so, was it a major or minor campaign for both parties? Thanks!
For almost the entire length of the Pacific War the Soviet Union and Japan were neutral towards each other. In April 1941 (two months prior to the German invasion of the USSR) the two countries had signed the Soviet-Japanese Neutrality Pact. The pact allowed each country to focus on their most immediate threat. Nazi Germany for the Soviets and the US/UK for Japan. Both countries strictly adhered to their neutrality. Japan permitted war materiel from the US to pass to the USSR through the Tsushima strait to Vladisvostok (under Soviet flagged vessels) without interference and the USSR interned the crew of one of the Dolittle raiders who landed in their territory. The two countries even continued to maintain embassies in each others capitals during most of this period.
At the Yalta conference between Stalin, Roosevelt and Churchill in February 1945 Stalin agreed to enter the Pacific War within three months of the end of the war in Europe. In April 1945 the Soviets announced that they would not be renewing the Soviet Neutrality Pact, though the pact did still have year to run.
As it became clear to the Japanese government and high command that they could not win the war they attempted to enlist the help of the Soviets to mediate a peace agreement with the Western Allies. However the Soviet government did not forward the request to the US and used the time to build up their forces facing Manchuria. On August 8th the USSR declared war on Japan and on the following day (coincidentally the same day Nagasaki was bombed & three months to day after Germany surrendered) the Red Army invaded Manchuria.
The Red Army forces, which did include some veteran units from the Eastern Front, were equipped with some of the best equipment fielded during the entire Second World War including around one thousand tanks. They were trained for fast maneuver warfare and used paratroops and amphibious landings to secure forward positions. The depleted & poorly equipped Manchuria garrison (the Kwantung army) was largely brushed aside with the Soviets taking just 11 days to secure the entire territory.
The Soviet invasion was a major shock to the Japanese government, as much as the atomic bombings were. With no major powers left to negotiate with they realized that they had no choice but to surrender.
Just a small addendum to what has been previously mentioned. The Soviets had begun to move down the Kuril Island Chain after their seizure of South Sakhalin, as part of the continuation of the Manchurian Strategic Offensive. There is evidence as well of Soviet plans (never executed due to concerns over allied response) to mount a landing in Hokkaido. Thus the Soviets presented a credible threat to the territorial integrity and inviolability of the Japanese home Islands, in addition to destroying desperately needed Japanese reserves, resources and industrial capacity in Manchuria. Given the state of Japanese defenses in the area the planned operation, though ramshackle by late war U.S. standards (though admittedly the Soviet navy gets short shrift for its demonstrated record of success in amphibious operations during the war), could very well have been a success and was of a sufficient scale to threaten Japanese control of Hokkaido (Airborne landings supporting the landing of two reinforced Rifle Corps). However credible the actual threat may have been, these Soviet amphibious landings against Japanese held islands were an unexpected, highly disconcerting turn of events for the Japanese leadership. While the Americans could be counted upon to approach Japan's well prepared southern defenses with caution, no such luxuries could be countenanced against the Soviets in the poorly prepared north. The thought of Soviet control over any part of the Japanese Home Islands was something unbearable to the leaders of the vehemently anti communist Imperial Japanese state. Leniency and the preservation of the emperor could at least be hoped for from surrender to the Americans.
A great account of the Soviet "Operation August Storm" https://www.google.com/url?sa=t&source=web&rct=j&url=http://usacac.army.mil/cac2/cgsc/carl/download/csipubs/LP8_AugustStormSovietTacticalAndOperationalCombatInManchuria_1945.pdf&ved=0ahUKEwj2z82Hv5XOAhVEx2MKHRU1Ak8QFgiIATAS&usg=AFQjCNFUiImIWk46ryMFer3KQgUkvGge5g&sig2=ocej_oa0Be7avrTZEtUZog