This isn't the best game of all time. It is really boring after awhile, and it is mostly grind,level up, take over city once it seems ok to take over, and repeat until all the nations are subdued.
Oh and pmaster no one enters a war for no reason it was pretty obvious that the french, and the british needed help since they were running out of able young men to fight the war and the germans were redeploying their troops in the eastern front to the west as russia was dealing with the Communist revolution, and so the US decided to help after The Allies have been asking for sometime already.
The reason there is no american soldiers is that the US entered very late in the war 1917(?) they were only there for about 10 months or so; compared to the british and the germans who have been fighting since 1914.