World War I, called the Great War until the occurrence of World War II, lasted from July 28, 1914 until November 11, 1918.
7th Regiment marching down 5th Avenue in New York City, circa 1916.
It involved all the major and economically developed countries, centering in Europe and spanning the globe from Africa to the Middle East as well as North and South America.
French soldiers look for enemy aircraft.
It started when the Austrian Archduke Franz Ferdinand was assassinated by Gavrilo Princip of Yugoslavia. Within weeks, the Allies and the Central Powers (namely Germany, Austria-Hungary, and the Ottoman Empire) had been roped into international war that eventually involved 70 million military personnel and killed over 9 million people.
A kiss before deployment. A World War I soldier goes to war.