Not quite true. The Russians had started to turn the tide of the war in the east before the US got involved.
The US was "involved" long before December 7, 1941.
The US began supplying Russia not long after Germany invaded in June 1941, and was paid for doing so. Lend lease aid begain in October 1941. The German advance was stopped in December 1941 but the situation remained exceedingly precarious for the Russians until Operation Uranus kicked off in November 1942. The US was already into the third phase of supplying aid to Russia by that time.