I know this is a topic of hot debate, so if there is another thread on this we can move the discussion there.
Some Americans claim that the US saved Europe, especially Britain, when the US entered into WWII. British people say this isn't true, but I have yet to see anything of substance as to why it isn't true. I'm no expert in WWII, but lets look at a few simple facts:
1. Britain did resist Nazi forces during Operation Sea Lion, but in the aftermath Britain was in no position to single handedly take back Western Europe from the Third Reich. If the US had stayed out of the conflict, Hitler could have regrouped and been able to conquer the UK.
2. Operation Overlord, the most important amphibious landing in the history of warfare would not be possible without the help of the United States. Without the industrial powerhouse that was the United States, the allies would not have had sufficient manpower and vehicles to mount a successful invasion.
3. After the closing of the European Theater it became evident that the Nazi's were hard at work on the atomic bomb and only months behind the Manhattan Project. If the United States had not entered the war when it did, Hitler could have developed the A-bomb first, which would have effectively made him the victor.
4. Some Brits point out that they stood alone against Nazi Germany before the US got involved. While this is true, Great Britain relied on supplies being sent from the United States as part of Roosevelt's lend-lease plan. Would you have been able to put up such a fight without American aid?
Now, not to say that Britain was helpless. Britain performed excellently and stood as a strong adversary to Hitler. But face it, if the US were to remain in isolation, Hitler would have won the war.