During World War I (1914-1918), called the Great War at the time, the United Kingdom joined the Allied Powers in fighting the Central Powers, which included Germany, Austria-Hungary, and the Ottoman Empire.

The assassination of the Austrian Archduke Franz Ferdinand by a Serbian nationalist on June 28, 1914, triggered tensions that had long been brewing among European nations. Germany, ruled by Kaiser Wilhelm II, backed Austria-Hungary in their declaration of war, while Serbia called upon Russia and its allies, including Great Britain, France, and Belgium. Some of the bloodiest fighting took place along the Western Front, located in France and Belgium, as the opposing sides attacked each other from trenches, a war of attrition that claimed millions of lives with very little gain on either side. At sea, engagements like the Battle of Jutland in 1916 proved the British navy superior to the Germans.

The composition of the Allied Powers changed in 1917, when Russia bowed out of the war due to an internal revolution as Vladimir Lenin and the Bolsheviks overthrew the czar, Russia’s absolute monarch, and installed communist rule. In the same year, President Woodrow Wilson brought the United States into the war on the side of the Allies. Facing internal revolts and economic collapse, the Central Powers reached an armistice with the Allies in November 1918.