@Mummelmann,
I hate to be the one to tell you this but the US was the biggest reason the Allied Forces won the war. By no means did the US do it alone, but as far as any single nation having an impact on the outcome, the US hands down has the biggest impact on that war.
(Red countries are Allied or Allied-controlled, Blue denotes Axis or Axis controlled countries, and the Soviet Union is colored Green prior to joining the Allies in 1941)
Watch this animated gif I got from Wikipedia, keep in mind the two important dates for when the US started giving support to the Allies. Dec '41 and June '44, the first date is obviously the 8th of Dec when the US formally joined the Allies and initial supply support was initiated. The second date is obviously Operation Overlord (aka Battle for Normandy) Which started on the 6th of June with the precursor invasion known as Operation Neptune (some of you played through this operation a bit in the BIA series). Both dates coincide with huge allied advancements in the months following that point. This should be a stark contrast to the constant Allied defeats happening prior to the the US joining the Allies.
There are a number of huge victories the US wasn't very involved with however. The best example I can think of off the top of my head is Operation Crusader (Nov/Dec '41) where the British 8th Army headed up by Allen Cunningham (not the poker player) finally defeated Erwin Rommel or some of you may remember him as the "Desert Fox". He had a legendary status amongst the troops in Africa and many thought he was unbeatable. It was in large part due to the cunning leadership of Cunningham that they were able to defeat him, another big factor was that the Allies were very effectively cutting Rommel's supply lines. But this was still before the US entered the war so no credit to them on this one.
There are countless situations involving every country where impacts were made. But the bottom line is that once the US entered the war the overall tide began to turn and it stayed turn.
PS - I am by no means a "History Buff", I have only read through the Wikipedia WW2 info a couple of times and I watch a large amount of the History Channel is all.