I had a relative (great-uncle?) who died in the war. But not in combat, instead during the massive 1918 influenza pandemic. He died just a few weeks after reaching Europe.
40% of American deaths in World War I were due to the influenza pandemic, which in fact killed far more people than the war did.
As late to the party as the USA was - something us Canadians like to point out a lot - the effect of American involvement was crushing to Germany; after all, Germany defeated Russia and effectively won the Eastern Front and was throwing a lot of those troops against the Western Allies. American manpower was overwhelming; the U.S. by mid-1918 was adding the equivalent of a new division of troops every two days. Germany, its army reinforced by men drawn from the East, still made a hell of a push to win the war, but it was simply too much for Germany to defeat when an entirely new Ally had been added to the mix.
Interestingly enough, the U.S. never formally declared war on the entire Central Powers alliance - it only declared war on Germany at first, Austria-Hungary later in 1917, and never declared war on any of their other allies at all. And then when the war ended the U.S. never ratified the Treaty of Versailles; it made peace separately.
My grandfather was juuuuust a little too young to be drafted for WWI, being born in October of 1917. If he had volunteered after that he would have been in service less than a year.
He was in Kansas City at the time, and told me about seeing trainloads of soldiers sick from the flu coming in. Also, the hospitals were full to overflowing. Luckily he didn’t catch it, nor did anyone else in my direct family line.
Baker your first sentence doesn’t make any sense. If he was born in 1917 he would have been one year old in 1918 and 2 years old in 1919.
The military made a major mistake returning these soldiers home when they were sick with the flu: they ended up spreading it to a lot more people than otherwise would have caught it.
The part of the family we have good track of were a bit too German to serve. 
Most of the boys of age trucked together in from their little still-German-speaking town to the Big city recruitment office,with the ones who spoke decent English translating for the others. In an curious twist, each and every one of those strapping young men who had been working 14 hour days on the farms and ranches since they were 8, had some completely-unnoticed-until-that-point medical issue that made them unfit to serve.
Germans young men and American young men of German heritage , well, it was not suprising that sometimes first cousins ended up shooting at each other.
Both of my grandfathers served. Neither went overseas. The unit of one of them was shipping out but he got the flu and by the time he was well the war ended. The flu was a big thing, esp. hard on young people. One grandparent lost two siblings to it.
They’re long gone, of course. But as of last year also gone are all the WWII vets in my family. Memories of all these things are rapidly disappearing.