Entering the fight — reluctantly
At first, the U.S. hoped to just send the Navy and let the Europeans do the ground fighting. The British and French quickly admitted their problem: They were almost out of men. The United States drafted a minimally trained army of 4 million men and began to ship them to Europe.
A year passed between the time America declared war and the time an effective U.S. fighting force assembled in Europe, and it was none too soon. Russia, which had been fighting on the Allied side, was swept by a Communist revolution and dropped out of the war.
Experienced German troops shifted to fight in France. By May of 1918, the Germans were within 40 miles of Paris. The first large American contingent was thrown right into a breach in the French line. In July, the German advance ground to a halt. By the fall, over a million American troops were helping to slowly push the Germans back.
Heroes came from the strangest places: Sergeant York, an American soldier raised in an antiwar church, singlehandedly killed 20 Germans and captured 132 more.
The Great War ends
In October, the Germans asked for peace based on Wilson's 14 Points. At 11:00 a.m. on the 11th day of the 11th month (November 11, 1918), the Great War was over.
The Allied forces won because Germany knew what was going to happen if it continued fighting; Socialist revolutions were going on back in Germany, and lots of German (and Allied) troops were dying from a worldwide flu epidemic. Germany had given up before it was completely defeated, something that would come to bother a hard-fighting, wounded corporal in the German army named Adolf Hitler.
The United States was far from the arsenal of democracy it would become in World War II; Britain and France actually supplied most of the planes, big guns, and transport ships used by American troops.
Leaders of the world hurried to Paris in January of 1919 to conclude a peace treaty while revolutions were tearing apart Russia and central Europe. Woodrow Wilson was the hero of the day. People expected freedom and peace from the 14 Points. Unfortunately, most of the points' good ideas didn't end up in the Treaty of Versailles, a compromise Wilson had to make with broke, tired, and angry European victors.
Wilson tried to move the world toward fairness, and he did succeed on getting a few new nations established and some more reasonable boundaries drawn. In the end, Wilson got a treaty with too much reparation money due to be paid by a too-poor Germany. He took what he could get to preserve his pet project, the League of Nations.