World War I was a global conflict fought between two coalitions, the Allied Powers and the Central Powers, from 1914 to 1918. Over 30 nations declared war during this period, and the majority joined on the side of the Allies, including Serbia, Russia, France, Britain, Italy, and the United States. The war was fought throughout Europe, the Middle East, Africa, the Pacific, and parts of Asia. The war was one of the deadliest in history, resulting in an estimated 9 million soldiers dead and 23 million wounded, plus another 5 million civilian deaths from various causes.

The causes of World War I were complex and multifaceted, but some of the key factors that contributed to the outbreak of war include nationalism, military alliances, and the assassination of Archduke Franz Ferdinand of Austria-Hungary by a Serbian nationalist in 1914. The war was also a war against people, and advances in weaponry and military technology provoked tactical changes as each side tried to gain an advantage over the other.

The war ended with the defeat of the Central Powers, mainly Germany, Austria-Hungary, and Turkey, by the Allies, mainly France, Great Britain, Russia, Italy, Japan, and the United States. The war was virtually unprecedented in the slaughter, carnage, and destruction it caused, and it forever altered the world’s social and political landscape. It led to the fall of four great imperial dynasties (in Germany, Russia, Austria-Hungary, and Turkey), resulted in the Bolshevik Revolution in Russia, and laid the groundwork for World War II.