It was known as “The Great War”—a land, air and sea conflict so terrible, it left over 8 million military personnel and 6.6 million civilians dead. Nearly 60 percent of those who fought died. Even more went missing or were injured. In just four years between 1914 and 1918, World War I changed the face of modern warfare, becoming one of the deadliest conflicts in world history.

World War I had a variety of causes, but its roots were in a complex web of alliances between European powers. At its core was mistrust between—and militarization in—the informal “Triple Entente” (Great Britain, France, and Russia) and the secret “Triple Alliance” (Germany, the Austro-Hungarian Empire, and Italy).

The most powerful