When Europe's armies first marched to war in 1914, some were still carrying lances on horseback. By the end of the war, rapid-fire guns, aerial bombardment, armored vehicle attacks, and chemical weapon deployments were commonplace. Any romantic notion of warfare was bluntly shoved aside by the advent of chlorine gas, massive explosive shells that could have been fired from more than 20 miles away, and machine guns that spat out bullets like firehoses. Each side did its best to build on existing technology, or invent new methods, hoping to gain any advantage over the enemy. Massive listening devices gave them ears in the sky, armored vehicles made them impervious to small arms fire, tanks could (most of the time) cruise right over barbed wire and trenches, telephones and heliographs let them speak across vast distances, and airplanes gave them new platforms to rain death on each other from above. New scientific work resulted in more lethal explosives, new tactics made old offensive methods obsolete, and mass-produced killing machines made soldiers both more powerful and more vulnerable. On this 100-year anniversary, I've gathered photographs of the Great War from dozens of collections, some digitized for the first time, to try to tell the story of the conflict, those caught up in it, and how much it affected the world.