All told, the Allied forces had a casualty rate of about 52 percent—22 million of the 42 million men sent to war.
The Central Powers lost 15 million of the 23 million men they mobilized, a 65 percent casualty rate.
In France these men were known as gueules cassées, men with broken faces.
Other soldiers bore no physical wounds but were devastated by what they had seen in war.
Empires were shattered, governments fell, and violent and destructive regimes came to power in several of the combatant countries.
Perhaps the only country to truly benefit from the war was the United States, which emerged as the world's greatest power.
The war had come at a time of unprecedented prosperity for Europe, and that prosperity and productive capacity were used to fuel a vast killing machine.
Once the killing stopped, Europe's economies did not return to their prewar expansion. Mass poverty among the working classes led to rapid inflation, and politicians could do little to stop it.
Several economists, however, have attempted a rough estimate.
Shortly after the war, the Carnegie Endowment for International Peace estimated that the war cost the world over 7 billion; a later estimate, quoted by Niall Ferguson in The Pity of War, sets the cost at 8 billion.