World History/Effects of the First World War
The First World War shattered the European political order. An entire generation of young men had perished in brutal trench warfare. The German, Russian and Austro-Hungarian Empires ceased to exist, and numerous national groups that had previously been under their control seized the chance to declare independence. The Allied Powers Britain and France, though victors, were exhausted and nearly bankrupt. The United States, which had kept Britain and France from collapsing by sending both armies and financial assistance, rapidly withdrew itself from European affairs after the conflict was over. Russia was convulsed by revolution, as Lenin's Bolshevik movement seized power and launched unrelenting hostilities against the economic "exploiters" of capitalism.
Germany was forced by the Treaty of Versailles to take the blame for the war, to pay massive reparations to the victorious Allies, and to cede territory to France and a restored Poland. The Bolsheviks briefly invaded Poland in 1920 but were defeated by Polish armies. The Austro-Hungarian Empire collapsed, with Czechoslovakia seceding, and other territories going to Poland, Serbia (which would become part of Yugoslavia), Italy, and Romania. These changes were made permanent in September 1919 by the Treaty of Saint-Germain, which also gave Yugoslavia the Austrian Adriatic Fleet. Bulgaria ceded small strips of territory to Romania, Yugoslavia, and Greece.
Most devastating, though, was a massive influenza outbreak that started on the front lines and spread throughout the world, carried by soldiers returning from the war. This outbreak killed more people in more countries than the war itself.