After the battle of Britain it was unlikely we would be invaded and lose the war, but we were never going to be strong enough to win it on our own.
Once Hitler invaded Russia and failed to capture Moscow, they were in the unenviable position of having to fight a war on two fronts. Once Russia gained the initiate on the Eastern front, it was inevitable they would lose, but we would then have had a Russian dominated Europe.
So what did the yanks do for us: They provided us and Russia with vital war equipment, but for us under lend-lease at the cost of our empire, and once they were actively involved with the fighting they helped to shorten the war.
So they help save Western Europe from Soviet domination. But if Roosevelt had taken the much tougher line that Churchill wanted at the Yalta Conference then Eastern Europe MAY not have been as dominated by the Soviets as it was.
Their participation in WWI was probably more critical to the allied success, where the Germans broke through the English / French line in a spring 1918 offensive into open country, and it required the US troops as a reserve to stem the tide. After this initial setback 1918 was a year of victories for the allies.