I've read quite a bit about this topic and I remember talking about it a little bit in some college history courses. It's sort of nuanced.
- World War II was perhaps the biggest catalyst to establishing America as a global power. Up until this point, the U.S. wasn't really looked at or viewed in this way, but the massive economic and manpower effort to enter the war basically helped catapult America into the power we know it as today. So, it did a lot of positive things for our global perception and how people viewed our contributions.
- We live in a America, so we tend to aggrandize our efforts (often unintentionally).
- America officially launched itself into the European war on D Day and it was over less than a year later, which I think further buoys the perceived value of our contributions, fair or not.
- Some people/countries like to take America's WWII success and use it as a dig or insult, sort of like uh 'oh yeah, of course Americans think they won the war,' or 'of course America thinks they did the most' but it really boils down to bad jokes and stereotyping.