A hundred years ago next week, in the small Balkan city of Sarajevo, Serbian nationalists murdered the heir to the throne of Austria-Hungary and his wife. People were shocked but not particularly worried. Sadly, there had been many political assassinations in previous years—the king of Italy, two Spanish prime ministers, the Russian czar, President William McKinley. None had led to a major crisis. Yet just as a pebble can start a landslide, this killing set off a series of events that, in five weeks, led Europe into a general war.

The U.S. under President Woodrow Wilson intended to stay out of the conflict, which, in the eyes of many Americans, had nothing to do with them. But in 1917, German submarine attacks on U.S. shipping and attempts by the German government to encourage Mexico to invade the U.S. enraged public opinion, and Wilson sorrowfully asked Congress to declare war. American resources and manpower tipped the balance against the Central Powers of Germany and Austria-Hungary, and on Nov. 11, 1918, what everyone then called the Great War finally came to an end.

(Take a look at the Journal's in-depth feature on 100 legacies from World War I that continue to shape our lives today.)

The cold numbers capture much of the war's horror: more than 9 million men dead and twice as many again wounded—a loss of sons, husbands and fathers but also of skills and talents. Graves in the north of France and Belgium and war memorials across the U.S. bear witness to the 53,000 American soldiers who died. Thousands of civilians died, too, during the war itself, whether of hunger, disease or violence. And then, as the guns were falling silent, a new pestilence struck humanity in the shape of a virulent influenza. As troops returned home, they unwittingly helped carry the disease around the world. It has been estimated that 50 million died.

Many of the now-familiar political boundaries in Europe and the Middle East still reflect the peace settlements that followed the war. These resulted in a smaller Russia and Germany and wound up the great multinational empires of Austria-Hungary and the Ottomans. New countries appeared on our maps, with names such as Yugoslavia and Iraq.