In the early 1900s tensions ran high in Europe. Nations embracing Imperialism competed for colonies. These nations had built large armies and allied with the countries around them to protect themselves. On June 28, 1914, in Sarajevo, Gavrilo Princip, a Serbian nationalist, shot Franz Ferdinand, the heir to the Austro-Hungarian throne. By the end of July 1914, this assassination had led to World War I, which involved most of the countries in Europe and many around the world. The two sides in the war were the Triple Entente and the Central Powers. The Triple Entente included Great Britain, France, and Russia, and the Central Powers included Germany, Austro-Hungary, and Turkey.

At first men excitedly enlisted to fight. But, when they reached the front lines, they quickly saw the reality of warfare. Soldiers fought and lived in trenches and holes dug in the ground. The trenches were wet and unsanitary and filled with rats and lice and other vermin. Many soldiers became ill from simply living in the trenches. The new weapons of war led to the deaths of many more. Machine guns, poison gas, submarines, airplanes, and tanks killed more soldiers more quickly than ever before.

Many countries, including the United States, tried to stay out of the war, but over time, many entered. In 1917, President Wilson declared war on Germany, saying “The world must be made safe for democracy.”

World War I caused 37 million casualties and cost over $180 billion before the Central Powers were defeated. The reparations imposed on Germany have been criticized for helping lead to further international crises, such as the rise of Nazism.