World War I: The Corfu Declaration, which leads to the creation of the post-war Kingdom of Yugoslavia, is signed by the Yugoslav Committee and Kingdom of Serbia.

World War I or the First World War, also known as the Great War, was a global conflict between two coalitions: the Allies and the Central Powers. Main areas of conflict included Europe and the Middle East, as well as parts of Africa and the Asia-Pacific. There were important developments in weaponry including tanks, aircraft, artillery, machine guns, and chemical weapons. One of the deadliest conflicts in history, it resulted in an estimated 30 million military casualties, plus another 8 million civilian deaths from war-related causes and genocide. The movement of large numbers of people was a major factor in the deadly Spanish flu pandemic.

The Corfu Declaration was an agreement between the prime minister of Serbia, Nikola Pašić, and the president of the Yugoslav Committee, Ante Trumbić, concluded on the Greek island of Corfu on 20 July 1917. Its purpose was to establish the method of unifying a future common state of the South Slavs living in Serbia, Montenegro and Austria-Hungary after the First World War. Russia's decision to withdraw diplomatic support for Serbia following the February Revolution, as well as the Yugoslav Committee's sidelining by the trialist reform initiatives launched in Austria-Hungary, motivated both sides to attempt to reach an agreement.

The Kingdom of Yugoslavia was a country in Southeast and Central Europe that existed from 1918 until 1941. From 1918 to 1929, it was officially called the Kingdom of Serbs, Croats, and Slovenes, but the term "Yugoslavia" has been its colloquial name as early as 1922 due to its origins. The official name of the state was changed to "Kingdom of Yugoslavia" by King Alexander I on 3 October 1929.