Below are the results of the same survey question that was asked in France over a 70-year period (1945, 1994, 2004 and 2015). They tell us a good deal about perception and reality over time.
The question that was asked of the French over time is real simple: "In your opinion, which nation contributed the most to the defeat of Germany in 1945?"
As you can see, in 1945, after the end of WWII much of France believed the Russians, who took the brunt of Germany's aggression, had more to do with the defeat of Germany.
However, over time, for any number of reasons, people have come to believe it was the United States who had more with defeating Germany during WWII.
Part of the reasons so many French in 1945 believed Russia had more to do with defeating the Germans was because of the large number of deaths - military and civilian - that the Russians experienced (about 23 million), which we can see from the graph below.
Here's the point. Our beliefs change over time, even if the facts and numbers don't. There's more to this story, but know that "who won" World War II is not necessarily an open and shut case.
You can access the article that raises the issue by clicking here (it's in French, but you can figure it out if you speak a Romance language, and with a little work).