I don't know if it was a sure thing for the Allies (and by Allies I mean Brito-America and not the Soviets). The Japanese controlled a large chunk of China, and all of Korea, Manchuko in the north and French Indochina (Vietnam etc) in the south. Up until the Japanese surrender the conflict was mostly a stalemate or slow Japanese advance. Even if Operation Downfall was successful (not doubt massively bloody) its very possible that the IJA would continue to fight on in China. The Russian August Storm offensive took Manchuko and and the Chinese were advancing in 1944 but that still left a huge stretch of China and most of Korea left (as the main Soviet force had yet to cross the Yalu when Japan surrendered) for the Japanese to defend. We are talking a region about the size of the Western Front in Europe.
The Soviets would have eventually overwhelmed the Japanese but the US would have to either invade from India (in to Nationalist Territory along one jungle/mountain road) or launch an amphibious invasion from the Philippines or conquered Japan (maybe invade Taiwan as a base) as the Nationalist Chinese had yet to take a coastal port. In that perspective it sort of becomes a Soviet Victory rather than an American one.
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