1917 was the decisive turning point. In Russia, the February Revolution toppled Tsar Nicholas II; the October Revolution brought the Bolsheviks to power. Lenin immediately concluded a separate peace with Germany (Treaty of Brest-Litovsk), ending the Eastern Front. That same year the United States declared war in response to Germany's unrestricted submarine warfare. Two million American soldiers tipped the balance decisively.

The 1919 Treaty of Versailles imposed sole war guilt on Germany along with 132 billion gold marks in reparations and 13% territorial loss. Four empires collapsed: Ottoman, Austro-Hungarian, Russian, and German. The resulting dozen new states became seeds of new ethnic conflicts. Economist Keynes warned the treaty would 'destroy Europe's economy' — and his prophecy proved accurate. Germany's humiliation and economic ruin gave rise to Hitler, leading to an even greater war 20 years later.