Margaret Macmillan at the WSJ:
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.