It wasn’t even that. The US got involved when it was clear the Soviets were going to win. Yes yes Pearl Harbor triggered it, but it’s well documented that the Japanese attack was something that the US strategists and FDR himself discussed as something they needed to find a way to cause, and they did it by blockading Japanese oil imports and forcing them to act. The US was never an actual ally to the USSR, they just needed to keep up those appearances so they could invade Europe to stop the Soviets from liberating the entire continent.
It wasn’t even that. The US got involved when it was clear the Soviets were going to win. Yes yes Pearl Harbor triggered it, but it’s well documented that the Japanese attack was something that the US strategists and FDR himself discussed as something they needed to find a way to cause, and they did it by blockading Japanese oil imports and forcing them to act. The US was never an actual ally to the USSR, they just needed to keep up those appearances so they could invade Europe to stop the Soviets from liberating the entire continent.