>>64086865
>The alternative was telling the Japanese military junta that Truman had made a secret agreement with Stalin at the Potsdam Conference that the USSR would declare war on Japan exactly three months after Germany's surrender.
The Japanese actually detected the buildup on the Manchuria border and suspected the Soviets were about to break the Non-Aggression Pact. Confirming their suspicions may not have actually made all that much of a difference.