I believe that the answer is that the use of nuclear weapons ended the war much faster than Stalin had anticipated. Stalin wanted to enter the war in the pacific, however the use of nuclear weapons crippled the Japanese, forcing them to surrender. Stalin was unhappy that his soldiers did not get to enter the war in the pacific, causing bad blood between Stalin and Truman. Eventually this bad blood lead to the Cold War as things got more tense between the two. Hope this helped!
