For the purposes of this article, a world war is a global conflict involving most or all major powers. The term is most commonly applied to two international conflicts that occurred in the first half of the 20th century—World War I (1914–1918) and World War II (1939–1945). Other global conflicts can also be considered world wars if they involved most of the world’s major powers, such as the Nine Years’ War (Prague), the Hundred Years’ War and the First Indochina War.

Throughout the 1900s, many of the world’s leading nations invested in making their armies more powerful and more capable. By 1914, most of them were ready to fight when Archduke Franz Ferdinand was assassinated in Sarajevo. The conflict would drag in other countries, including Russia and Germany, on opposite sides and eventually become a worldwide affair.

The United States was initially neutral, but public opinion began to shift after the sinking of the Lusitania by German U-boats in May 1915, killing hundreds of American sailors. When Russia’s political revolutions pushed it into the war against Germany, President Woodrow Wilson appeared before Congress and called for a declaration of war.

Over the next two years, millions of Americans were inducted into military service as the country entered the war on a global scale. This was the first time a major war was fought on so many fronts, from the trenches of France and Belgium to strategic bombing campaigns against civilian targets. Long periods in the front lines without relief, low wages and shortages of food and supplies prompted thousands of troops to mutiny.