The world was shaken in the first great conflict to take place since the end of the Napoleonic Wars. Millions of people died from hunger, disease, starvation and brutal slaughter on the battlefield or in prison camps and concentration camps. The causes of the war remain a subject of bitter debate, though there is general agreement that national self-interest was a major factor. In July 1914, the assassination of Franz Ferdinand in Sarajevo triggered a series of political misjudgements and fear of losing prestige that resulted in the entanglement of all the major powers in a global conflict whose consequences continue to reverberate.

The great powers of Europe were defeated and the Austro-Hungarian, Russian, and Ottoman empires ceased to exist. The Treaty of Versailles imposed onerous reparations on Germany and established the League of Nations to prevent future world conflicts. Resentment over the terms of the Treaty fueled the rise of Adolf Hitler in Germany and set the stage for the second world war.

By the end of the war, the world had transformed. Modern weaponry like tanks, aircraft and machine guns had revolutionized warfare. The influx of millions of new recruits, including women, had altered society in many ways. Nationalism grew and the United States emerged as a superpower. The resulting tensions with the Soviet Union and Japan launched the Cold War that led to a half century of skirmishing and nervous watchfulness as two rival blocs, each armed with nuclear weapons, tested each other for weakness.