The history of war is inseparable from the history of humanity. It is impossible to understand the world today without first understanding how war has shaped our lives. The Russian Revolution proliferated communism. The American Civil War ended slavery. World War II shifted the global balance of power. From the Crusades to the Mongol Conquests, from the French Revolution to the Vietnam War—wars can expand and dismantle empires; end nations and birth new states; claim untold lives, erase cultures and peoples, swallow up families, communities, and even entire civilizations. But war can also stimulate scientific innovation, earn captive people the right to be free, and show the potential for courage, principle, and heroism in ordinary people.