“especially if Japan did attack the Soviets”
Japan had no interest in attacking the Soviet Union. The Soviet Far East was sparsely populated and undeveloped. The only real city was Vladivostok which while critically important to the Soviet Union was of no interest to Japan as long as the USSR stayed neutral. Stalin had to understand that Vladivostock was indefensible and agreed to a nonagression pact with Japan.
Stalin broke that pact on the morning of August 9, 1945. A few hours later, the second atomic bomb was dropped, on Nagasaki. The Soviet Army would in the next two weeks overrun an area the size of all of Western Europe, defeating the formerly terrifying Kwantung Army whose reputation for cruelty rivaled that of the SS.