legit response-
I was taught after the assassination of the arch duke Franz Ferdinand by the terrorist group the black hand who was secretly funded by the Serbian govnt. the Austrian-hungry countries attacked serbia serbia causing a chain reaction of countries choosing sides due to their alliances. America trying to stay out of Europe's back and forth bull shit attempted to stay out of the war. After the un-instigated attack on the lusitania and the deaths of some american civilians the Us joined the war to help the Allies. (the us was not completely innocent though it had been profiting off the war selling weapons to the allies) the US not being a big competetior at the time in European business was not given much credit for their soldiers at the time of their entry into the war. After the us performance in the battle at belleau many countries were impressed with what they had and they were considered fearless (how the marines got the nickname "devil dogs") Germany being a bunch of dummys started a 2 front war that they could not win and eventually being withered away to nothing had to surrender signing the treaty of Versailles.
Boom that is all from memory and 3 years away from when i learned it so much of it may be wrong but i am prettty sure i got the jist of it...