It took the Russians to defeat Germany. They didn't need America coming in on the west. Britain could never have done it. It became, in effect, an American aircraft carrier, but they could not have taken Germany alone either. Only Russia could have done it alone
You need to look again at history of WW2.
Germany had a pact with the Russians, they weren't interested in helping defeat Germany.
Germany then attacked the Russians, in one of the most stupid military campaigns ever but even so, swept through the Red Army, but winter and Germany unable to re supply, led to the Russians re grouping and inflicting huge cruelty on the Germans, (understandable but brutal)
The USA coming into the war, both against Japan and Germany, would have defeated Germany regardless and had the Russians continued fighting, them as well.
They had nuclear weapons and weren't afraid to use them, in 1945, no one else did.
The numbers of planes tanks ships etc being produced by the US in 1944/45 was beyond belief, everything was better than what Germany or Russia had.
The USA has been guaranteeing our security ever since and even more today.