The idea of an independent South Slav state advanced during World War I, especially after Bolshevik Russia disclosed the secret 1915 Treaty of London, in which the Entente had promised to award Istria and much of Dalmatia and the Slovenian lands to Italy. Because they feared Italian domination, Ante Trumbic and other Dalmatian leaders formed the London-based Yugoslav Committee to promote creation of a South Slav state. In July 1917, Nikola Pasic of Serbia and Trumbic signed the Declaration of Corfu, which called for a union of Serbs, Croats, and Slovenes in one nation with a single democratic, constitutional, parliamentary system, under the Karadjordjevic Dynasty. The declaration promised equal recognition of the Latin and Cyrillic alphabets, the three national names and flags, and the three predominant religions. However, it did not indicate whether the new state would be centralized or federal. Pasic advocated a centralized state; Trumbic pressed for a federation.

The authority of Austria-Hungary over its South Slav lands ended in October 1918, and a National Council of Slovenes, Croats, and Serbs became the de facto government of the regions, under Antun Korosec. On October 29, the Sabor in Zagreb annulled the union of Croatia with Hungary and gave the National Council supreme authority. In November Pasic, Trumbic, and Antun Korosec signed an agreement in Geneva, providing for a joint provisional government but recognizing the jurisdiction of Serbia and the National Council in the areas under their respective control, until a constituent assembly could convene. But the war ended very rapidly, and Italy began seizing parts of Dalmatia. This prompted the National Council to seal a quick final agreement with Serbia, over the objections of the Croatian Peasant Party, without obtaining guarantees of regional autonomy. Leaders in Bosnia and Hercegovina and Vojvodina favored union; on November 24, the Montenegrins deposed the Njegos Dynasty and declared solidarity with Serbia. On December 1, Prince Regent Aleksandar Karadjordjevic and delegates from the National Council, Vojvodina, Bosnia and Hercegovina, and Montenegro announced the founding of the Kingdom of the Serbs, Croats, and Slovenes, to be ruled by Aleksandar. The Paris Peace Conference recognized the kingdom in May 1919.

Data as of December 1990