Actually, it wasn't that quick. U.S. communists were so micromanaged by Stalin, it took a couple weeks. Because Stalin was so depressed by Hitler's betrayal, he holed up in his dacha. U.S. communists were still calling FDR an imperialist until word came down from Stalin.
That may or may not be true, but the point is exactly the same. That very dynamic was what drove Whittaker Chambers into the arms of the Free World.
It was more convoluted than that.
William R Davis*, funneled nazi money to numerous anti war groups who were trying to keep the U.S. out of the war in Europe.
These groups such as the “No Foreign War Committee” and “America First Committee” were being funded by both the nazis and the soviets.
The soviets tried to take over the AFC, and when Barbarossa happened, the soviets quickly labeled the AFC as nazi collaborators.
*William R Davis’s grandson would later become the governor of California BTW.