1 December 1918: the Kingdom of Serbs, Croats, and Slovenes Proclaimed

Ensign of the New Kingdom

The idea for the unification of the southern Slavs emerged in the 19th century, and the strength of its appeal varied over the course of its development. During the First World War, unification became the main war aim of the government of the Kingdom of Serbia as well as the Yugoslav Committee, the group of exiled Habsburg Croat, Serb, and Slovene politicians and intellectuals  formed in Italy and based in London. In different ways, these two groups advocated for Yugoslav unification. 

Serbia suffered over a million dead during the Great War: 450,000 military and 650,000 civilians. It was, however, on the winning side, and by the time of the Armistice, its combat forces—deployed mainly in the Salonika sector—were rebuilt. Despite terrible suffering at home during the enemy occupation, at war's end, Serbia was the local "Strong Man." With the disintegration of the Habsburg Empire at the end of the Great War in 1918, many of the empire’s southern Slav minorities sought the protection of the Serbian throne and entered into union with Serbia as the Kingdom of Serbs, Croats, and Slovenes on 1 December 1918. 

Nikola Pašić, Serbian Nationalist, Many-Times Prime
Minister of  Serbia and the New State

On 7 February 1919, the United States recognized the Kingdom of the Serbs, Croats, and Slovenes through a statement released to the press by the U.S. Acting Secretary of State Frank Polk. The United States considered this new state as the successor state to the Kingdom of Serbia. The Paris Peace Conference subsequently underwrote a new pattern of state boundaries in the Balkans. The various treaties  established the right to self-determination as had been defined in January 1918 by President Wilson in his Fourteen Points. This right became a key criterion for the political order in both East-Central Europe and Southeast Europe. Every people was to be free to create a nation state of its own, provided that certain language and ethnographic criteria were met. Economic, historical, and strategic factors also played a role. 

With the peace agreements of 1919/1920, the Great Powers created a corridor stretching from the Baltics to the Balkans of nation states that had liberal democratic constitutions and welfare state systems. These were to act as a cordon sanitaire against revolutionary Bolshevik Russia, on the one side, and revisionist Germany on the other.

The major beneficiary there was the newly created Kingdom of Serbs, Croats, and Slovenes, which comprised the former kingdoms of Serbia and Montenegro (including Serbian-held Macedonia), as well as Croatia, Bosnia and Herzegovina, Austrian territory in Dalmatia and Slovenia, and Hungarian land north of the Danube River. 

Peter I was King of Serbia from 1903 to 1 December 1918,
when he became King of the Serbs, Croats, and Slovenes.
He held that title until his death three years later.

In 1929, after a decade of acrimonious party struggle, King Alexander I in 1929 prorogued the assembly, declared a royal dictatorship, and changed the name of the state to Yugoslavia. The historical regions were replaced by nine prefectures (banovine), all drafted deliberately to cut across the lines of traditional regions. None of these efforts reconciled conflicting views about the nature of the state

In 1946, Yugoslavia became a socialist federation of six republics: Croatia, Montenegro, Serbia, Slovenia, Bosnia and Herzegovina, and Macedonia. At this time, it adopted the name Socialist Federal Republic of Yugoslavia (SFRY). On 25 1991, Slovenia and Croatia declared their secession from the Yugoslav federation. Macedonia (now North Macedonia) followed suit on 19 December, and in February–March 1992 Bosniaks (Muslims) and Croats voted to secede. As civil war raged, Serbia and Montenegro created a new federation, adopting a new constitution on 27 April 1992.  

An agreement, ratified in 2003, renamed the country Serbia and Montenegro and effectively consigned the name Yugoslavia to the annals of history. Serbia and Montenegro was dissolved on 3 June 2006, when Montenegro declared its independence.

Sources: Office of Historian, U.S. Department of State; Encyclopedia Britannica; History of Yugoslavia, Purdue University Press, 1914-1918 Onlne.