World War I lasted four years, three months and two weeks, beginning in July 1914 and ending in November 1918. The conflict started with Austria-Hungary declaring war on Serbia in retaliation for the assassination of Archduke Franz Ferdinand.
The war featured the Central Powers of Austria-Hungary, Bulgaria, Germany and the Ottoman Empire (Turkey) against the Allied Powers of Belgium, France, Great Britain, Greece, Italy, Montenegro, Portugal, Romania, Russia, Serbia and the United States. The war resulted in an estimated 8.5 to 9 million military and 7 million civilian casualties. Peace was restored when Germany agreed to the armistice terms proposed by the Allies.