The heir to the Austro-Hungarian throne was assassinated in Sarajevo, and by summer 1914 Europe was like a barrel of gunpowder. With a growing number of nations with large militaries and opposing military alliances, the smallest spark could set off world war.

Militarism—a belief that the military was God’s gift to civilization—fueled Europe’s tensions. Countries sought to outdo one another in weapons development, resulting in an arms race. Moreover, European leaders were resentful of US President Woodrow Wilson’s Fourteen Points, which he hoped would guide peace negotiations.

The resulting conflict killed more people—9 million soldiers, sailors, and flyers and 5 million civilians—and cost more money than any other war before it. It also introduced new methods of warfare, such as airplanes, tanks, long range artillery, submarines, and poison gas.

Invading armies unleashed horrific violence against the people of the occupied areas. This shattered popular support and reduced the chances for peace. Many nations were pushed to their limits of endurance and collapsed. Amid the suffering, millions of volunteers and conscripts served in mass citizen armies. They were joined by millions of others who supported the war effort through industry and agriculture. Propaganda demonised enemy peoples and encouraged racial hatred. The world watched with bated breath as Europe marched toward a bloody climax. By the end of the war, more than 65 million men had volunteered or been conscripted to fight. In addition, hundreds of million more contributed to the war effort by working at home or supporting their loved ones who fought in the army.