The US alliance with Stalin came AFTER the Nazi-Soviet pact.
The fact that capitalists needed communists to help win the war
You have it backward. Stalin needed US aid to save his country from Nazi domination. We provided Russia their ammo, their trucks, their planes, all their equipment.
Excerpts from rferl.org
"I want to tell you what, from the Russian point of view, the president and the United States have done for victory in this war," Stalin said. "The most important things in this war are the machines.... The United States is a country of machines. Without the machines we received through Lend-Lease, we would have lost the war."
Nikita Khrushchev offered the same opinion.
"If the United States had not helped us, we would not have won the war," he wrote in his memoirs. "One-on-one against Hitler's Germany, we would not have withstood its onslaught and would have lost the war. No one talks about this officially, and Stalin never, I think, left any written traces of his opinion, but I can say that he expressed this view several times in conversations with me."

Most visibly, the United States provided the Soviet Union with more than 400,000 jeeps and trucks, 14,000 aircraft, 8,000 tractors and construction vehicles, and 13,000 battle tanks.
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More info:
https://share.america.gov/america-sent-equipment-to-soviet-union-in-world-war-ii/
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