The first world war ended in 1945 with the defeat of Germany, which declared war on the USA on 11 December 1941. The war was fought primarily by naval forces, although the United States had to make use of land-based armed forces, too. After the Japanese attack on Pearl Harbor, the U.S. began to provide increased economic assistance to Britain and the Soviet Union. However, the resulting war was a major setback for both nations.

World War I began as a long-simmering conflict between European nations in 1914. The assassination of Austrian Archduke Franz Ferdinand by a Serbian nationalist group known as the Black Hand triggered an all-out war between the Central Powers and the Allies. The Central Powers included Germany, Austria-Hungary, Bulgaria, and the Ottoman Empire, while the Allies were the United States, Great Britain, Italy, and Japan. Amid the fighting, the United States entered the war, motivated by the threat of German submarines.

The first world war was unprecedented in its length and cost, with more than 9 million people killed. It involved 28 countries, costing approximately 186 billion dollars in direct costs and $151 billion for indirect costs. During the war, tanks, airplanes, long-range artillery, and submarines made their first appearance. Moreover, the use of poison gas and airplane-dropped bombs were new concepts. In addition, seven million men died or were permanently disabled.