The American involvement in World War I was one of the greater U.S. foreign policy mistakes of the 20th century. Instead of recognizing it for what it was - a war between imperialist European powers - America in 1917 entered the war on the premise that it would end all wars.
Of course, things turned out otherwise. President Woodrow Wilson's idealism, as exemplified in the 14 Points, proved to be devastating. Wilson and many pragmatist American intellectuals like John Dewey believed they could bring democracy to Europe. But as should have been clear from the beginning, European countries were not willing to follow the 14 Points and give up any advantages arising from their victory over Germany and Austria-Hungary. According to Randolph Bourne, Americans were not even sure what they meant by "democracy" when they thought they were promoting it abroad.
In 1919, Wilson's 14 Points, which had enticed Germany and Austria-Hungary to negotiate in 1918, were more or less sacrificed to the interests of France and Great Britain. The Treaty of Versailles constituted a major setback for American foreign policy, which was outwitted by European diplomats. In hindsight the treaty's material effects (substantial territorial losses and reparation payments) were hard-hitting, but bearable. However, the emotional effects on the German people were traumatic and partially paved the way for the Nazi movement. The Versailles Treaty also inevitably associated the democracy of the ill-fated Weimar Republic with the stain of defeat in the consciousness of millions of Germans.
In effect, America in World War I proved to be a pawn to Great Britain and France. Without American involvement, the war might have resulted in a more balanced outcome.
A domestic effect of U.S. involvement in World War I was the effective destruction of the lively German-American culture by censorship and government-sponsored propaganda. Ironically, Woodrow Wilson won the presidential elections of 1916 on the premise that he would keep America out of war. Only months after Wilson's being sworn into office, America was entangled in the war.