The immediate trigger for World War I was the assassination of Archduke Franz Ferdinand of Austria, the heir to the Austro-Hungarian throne. On June 28, 1914, Ferdinand and his wife, Sophie, were visiting the city of Sarajevo in Bosnia-Herzegovina, which had been annexed by Austria-Hungary in 1908. They were traveling in an open car when they were shot by Gavrilo Princip, a Bosnian Serb nationalist.
Austria-Hungary's Ultimatum to Serbia
Austria-Hungary blamed Serbia for the assassination and issued an ultimatum demanding that Serbia punish those responsible and take measures to prevent future anti-Austrian activities. Serbia agreed to most of the demands but refused to allow Austrian police to investigate the assassination on Serbian soil.
Austria-Hungary Declares War on Serbia
On July 28, 1914, Austria-Hungary declared war on Serbia. Russia, as Serbia's ally, began mobilizing its military in response. Germany, which had a mutual defense pact with Austria-Hungary, declared war on Russia on August 1. France, which was allied with Russia, declared war on Germany on August 3.
The Schlieffen Plan
Germany had developed a war plan called the Schlieffen Plan, which called for a swift invasion of France through Belgium and Luxembourg. The plan was designed to quickly defeat France before Russia could fully mobilize.
The Battle of the Marne
The German advance through Belgium and Luxembourg met with strong resistance from the French and British armies. The Battle of the Marne, fought from September 6-12, was a turning point in the war. The French and British were able to stop the German advance and force them to retreat.
The Stalemate on the Western Front
The war on the Western Front quickly settled into a stalemate. Both sides dug in along a long line of trenches and fought a bloody war of attrition. The fighting continued with little change until the end of the war in 1918.
The Eastern Front
On the Eastern Front, the Russian army was initially successful against the Germans and Austro-Hungarians. However, the Germans were able to turn the tide of the war in their favor in 1915 and occupy large swaths of Russian territory.
The United States Enters the War
The United States entered the war on April 6, 1917, after the sinking of the Lusitania, a British passenger ship, by a German U-boat. The United States provided fresh troops and supplies to the Allied Powers and helped to turn the tide of the war in their favor.
The Armistice of 1918
On November 11, 1918, an armistice was signed between the Allied Powers and Germany, officially ending World War I. The war had lasted for four years and had claimed the lives of millions of people.