What evidence could a prosecutor use "against" Stalin to prove that he was mostly responsible for the the bombs on Hiroshima/Nagasaki?

Arguing in favor of Stalin’s responsibility for United States dropping atomic bombs on the Japanese cities of Hiroshima and Nagasaki is a tricky issue, specifically because Stalin’s responsibility in the matter, insofar as it can be determined, does not rest in his direct involvement in the events. That is not to say that Stalin is not at least partially responsible for the actions of the United States against Japan in August 1945.

The most revealing factor in this equation is the generally uneasy relationship between the Soviet Union and the United States during World War II. Since Russia had pulled out of World War I after the October Revolution in 1917, the United States was cautious in its dealings with the Communist government that had replaced the Czar. This uneasiness and caution was exacerbated by Stalin’s actions near the end of World War II. As the Allies began to retake those lands to which Germany had laid claim in the years leading up to the war, the Soviet Union seized the opportunity to create satellite states from the countries of Eastern Europe. The United States, perceiving Stalin’s actions to be a repeat of Hitler’s aggression, feared that Communism would spread throughout the rest of Europe. It has been argued that this fear prompted Harry S. Truman to drop the bombs on Hiroshima and Nagasaki. Detonating the bombs was a show of force meant to send Stalin a message: if you keep doing what you are doing, this could happen to you! The Soviet Union being the central source for American fears prompted the United States to commit these two acts.

A less compelling argument for what transpired is that the Soviet Union failed to commit to armed action against Japan until Germany was defeated. The United States, forced to fight on separate fronts and not receiving adequate military aid from the other Allies, could not wait for the conclusion of affairs in Europe; the loss of life in the fight to take Japan had simply taken too much of a toll. Fearing that they could not hold out for Soviet aid, Truman perceived the use of the atomic bomb as the only viable option left open to him. If the Soviet Union had committed to aiding the American campaign against Japan, the use of the atomic bombs may not have proven necessary. While Stalin did not push the button to drop the bombs, his actions, or perhaps his lack of action, certainly affected American military action against Japan.