World War I started in 1914 and ended in 1918. At first, people called it the Great War and thought it would end all wars. It was the biggest war the world had seen. Most of the fighting was in Europe and the Middle East. Over 8 million soldiers and sailors died, and more than 20 million were hurt. Many people thought there would never be another war after this one. However, the things that happened during World War I led to another big war, World War II, which started in 1939.

The war started when a prince was shot in a city called Sarajevo. Sarajevo was in a place called Bosnia, which was part of a big empire called Austria-Hungary. Austria-Hungary and its neighbor Serbia didn’t get along. Some Serbians wanted their country to be free from Austria-Hungary's rule. On June 28, 1914, a Serbian shot and killed a prince from Austria-Hungary. Austria-Hungary said Serbia planned it.

On July 28, 1914, Austria-Hungary declared war on Serbia. Both countries asked their friends for help. Austria-Hungary’s friends were Germany and Italy. They were called the Central Powers. Serbia's friends were Russia, France, and the United Kingdom (Great Britain). They were called the Allies.

In 1917, there was a big change in Russia called the Russian Revolution. Russia made peace with the Central Powers and left the war. Then the United States joined the Allies. President Woodrow Wilson didn’t want to go to war at first, but he changed his mind because Germany kept attacking U.S. ships. The United States also found out that Germany had promised part of the U.S. to Mexico if Mexico helped in the war. So the United States declared war on Germany on April 6, 1917.

During the war, both sides tried new ways to fight. They used poison gas and built tanks and airplanes for the first time. Germany bombed the United Kingdom with zeppelins, which are big floating aircraft. Later, both sides built airplanes for fighting and bombing. The war changed how people fought wars forever.

Source: World War I
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