World War I had far-reaching consequences in a variety of areas, but the most prominent was the near-destruction of Germany and the direct cause of World War II. Other consequences include the downfall of the Russian tsar and the loss of many European colonies on other continents. Finally, it allowed the United States to take its place on the world stage of geopolitics.
The Treaty of Versailles marked the formal end of the war, but one of the conditions of surrender was that Germany take total blame for the war and pay all reparations. This caused the German people and government to feel resentful and suffer hyperinflation. Adolf Hitler took advantage of these feelings of resentment to form a dictatorship and annex territories such as Poland and the Sudetenland.
Russia experienced near-total war and suffered poverty. The Bolshevik Revolution successfully overthrew the Romanov royal family in 1917, giving rise to Vladimir Lenin's government and the establishment of a socialist state.
Europe was embroiled in a war of attrition from 1914 to 1918, losing the majority of young adult men and a great deal of resources. This made it easy for the European powers to lose their hold on various colonies. Meanwhile, the United States entered the war late, suffered comparatively little loss and could assert itself as a world power.