Joachim von Ribbentrop, the German foreign minister, and his Russian counterpart, Vyacheslav Molotov, had negotiated the agreement and given their names to it. Its essence was that the Nazis and Soviets would not attack each other. The agreement also provided for the invasion and division of Poland between the two countries.
But Hitler put an end to the Molotov-Ribbentrop pact in June 1941 by launching Operation Barbarossa – the attack of the Soviet Union. The Russians were now in a fight for survival. And so they duly joined the alliance that had come together to fight the Germans.
The U.S. was a little slower in joining the Allies in the fight to crush Hitler and the Germans. But America had actually ended its policy of neutrality before the Soviets did. After attacks from German submarines on American shipping took place, neutrality was ditched in March 1941.