The outbreak of a world war in 1914 was the culmination of long-simmering tensions between nations. The assassination of Archduke Franz Ferdinand of Austria by a Bosnian Serb nationalist, Gavrilo Princip, was the trigger. Soon, countries that controlled territory on six continents were at war. The war was fought between what came to be known as the Triple Entente—Great Britain, France and Russia—and the Axis powers of Germany, Italy and Japan.

The conflict was the first to involve most of the world’s nations, and it was a brutal one, with huge casualties for both sides. Over 13 million military personnel died—including more than nine million in Europe, and an estimated three-quarters of a million in the Ottoman Empire. Some historians believe that the total number of civilian deaths is even higher—perhaps up to 20 million.

After the war, aggressive nationalism emerged in parts of the world, including the United States and the Soviet Union, as well as Germany and Fascist Italy. In 1938, Germany, in violation of the treaty of Versailles, annexed Austria and Czechoslovakia. Japan sought to acquire colonies in Asia to secure natural resources and markets for its industrialized economy.

The war also introduced new technologies such as tanks, battleships and military aircraft. As a result, women began to enter the workforce in large numbers, and interest in intergovernmental organizations grew. The United States became the world’s leading economic power and its navy greatly strengthened with a decisive victory at Midway.