I don't mean to belittle the Brit's determination in battle, but you might want to consider the timeline on American contributions. Furthermore, your Air Force was decimated, and your Navy wasn't technologically sufficient to defend your homeland. Newsflash. The English Channel isn't that big. I hear you have a tunnel now.
With the greatest respect, you are wrong. Our navy was very technically profificient - our invention of radar early on saw to that. It was certainly technologically sufficient to keep the entire Nazi surface fleet in harbour for the whole war with a couple of notable exceptions. Our airforce was not decimated, it was stronger immediately after the defeat of the luftwaffe than it had ever been and we were knocking the krauts out of the sky 3 for 1. We had radar you see, and no-one else did including you. The English channel is plenty big enough as a 1000 years of failed invasions, including the nazis bear testament to. If in another 800 years the US has never been successfully invaded, get back to me! :)