World War I began on July 28, 1914. The war was triggered when Austria-Hungary declared war on Serbia, a result of the assassination of Archduke Franz Ferdinand one month earlier, also known as the Shot Heard Around the World.
Austria-Hungary was joined in the world's first global conflict by Central Powers Bulgaria, Germany and the Ottoman Empire against the Allied Forces of the United States, Great Britain, France, Russia, Italy and Japan. The 1916 Battle of Verdun in France was the longest in the war, resulting in nearly one million casualties. By the time fighting ceased in November 1918, more than nine million soldiers were killed. The Treaty of Versailles officially ended the conflict between Germany and the Allied Powers on June 28, 1919.