Take a look at the numbers. Without the US, Germany would've owned Europe.
The lend lease program allowed Britain and the USSR to stay in the war long enough for the US to get involved. Well over half of all the equipment used by the Soviets during WWII came from the US, and that number is even greater for Britain. How many US solders landed at Normandy compared to British soldiers? Even with the 8th airforce, the bombing campaign in Europe was almost called off due to the large number of planes lost over Germany, imagine how long it would've lasted had over 3/4 of the aircraft used not been there. And what would've happened in N. Africa had Operation Torch not occurred?
Without US involvement, the Germans were assured a fairly easy victory.