The world’s powers splintered into two opposing military alliances. The Allies were led by the British Empire, the Soviet Union and the United States; they fought against the Axis powers of Germany, Italy and Japan. They were squabbling over control of the planet’s resources and wealth, but they also sought to assert their regional dominance. They had more overall population, industrial and military strength than their rivals. In theory, this meant that they should be able to deter attack from stronger nations by promising to support the weaker ones.
But, in the real world, the alliances tended to expand local quarrels into continent-spanning conflicts. The killing of Franz Ferdinand and his wife, Sophie the Duchess of Hohenburg, by Bosnian Serb nationalist Gavrilo Princip in Sarajevo, was no exception. In response, the heirs of Austria-Hungary were crowned as king and queen of Serbia; Russia intervened to support its client state in the Balkans. Soon German U-boats were sinking neutral ships in the North Atlantic, and, on April 2, President Woodrow Wilson appeared before Congress to request a declaration of war against Germany.
This is a really bloody war, and by the end of it, no one was happy. Britain lost a quarter of its people; Germany suffered enormous losses, including defeat at Stalingrad and a humiliating treaty of surrender that slashed its economy and deprived it of its former colonies in exchange for a reunified, more modestly-sized nation that now included Romania, Bulgaria, Greece and the new country of Yugoslavia.