When war began in 1914, few expected a global conflict that would last for more than four years and leave a generation scarred by its brutality. In the first days, armies relied on outdated methods of communication such as carrier pigeons and could only hope to defeat each other with old-fashioned machine guns and artillery. But as the conflict wore on, innovations such as submarines, airplane-dropped bombs, and poison gas transformed warfare.

By the end of the war, more than twenty countries that controlled territory on six continents had declared war: Britain, France and Russia formed the Triple Entente; Germany, Austria-Hungary, and Italy joined to form the Central Powers; and Japan and the United States became the Allies. Many of the nations involved were colonies of the empires that started the war, generating discontent and resentment among people who felt they were being exploited.

As the deadly fighting raged across Europe, Africa, and Asia, young men came of age on the frontlines and learned lessons that would shape their lives—and the world’s. Some of them would rise to become heroes, forged in courage under fire; others would emerge as the most feared villains in history. This is the story of their 30-year struggle—the world’s most deadly war.