The United States was untouched by the horrors of WWI. European countries, however, had the task of recovering from the war since it was fought on their soil. Plus, the 1918 Spanish Flu pandemic made recovery worse. Germany, especially, had it bad. The terms of the Treaty of Versailles blamed Germany solely for starting the war- which led to lots of strict reparations and military limits. European colonies were also getting more discontent. Many had served in the war, forcibly or voluntarily, so they expected independence for their service. The United States experienced a cultural revolution and economic boom after the war. Ever heard of the "Roaring Twenties"? The rest of the world- not so much.
In general it was the "slum dwellers" and the "immigrants" who did not share in the prosperity of the late 1800s, since these people were paid very low wages and treated poorly.
