Alexander I of Yugoslavia in the context of "Principality of Montenegro"

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" (lit. 'Land of the South Slavs') 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.

The preliminary kingdom was formed in 1918 by the merger of the provisional State of Slovenes, Croats and Serbs (itself formed from territories of the former Austro-Hungarian Empire, encompassing what is now Bosnia and Herzegovina and most of what are now the states of Croatia and Slovenia) and Banat, Bačka and Baranja (that had been part of the Kingdom of Hungary within Austria-Hungary) with the formerly independent Kingdom of Serbia. In the same year, the Kingdom of Montenegro also proclaimed its unification with Serbia, whereas the regions of Kosovo and what is today North Macedonia had become parts of Serbia prior to the unification.

Yugoslavia (/ˌjuːɡoʊˈslɑːviə/; lit. 'Land of the South Slavs') was a country in Central Europe and the Balkans that existed from 1918 to 2003. It came into existence following World War I, under the name of the Kingdom of Serbs, Croats and Slovenes, from the merger of the Kingdom of Serbia with the provisional State of Slovenes, Croats and Serbs, and constituted the first union of South Slavic peoples as a sovereign state, following centuries of foreign rule over the region under the Ottoman Empire and the Habsburg monarchy.

Under the rule of the House of Karađorđević, the kingdom gained international recognition on 13 July 1922 at the Conference of Ambassadors in Paris and was renamed the Kingdom of Yugoslavia on 3 October 1929. Peter I was the country's first sovereign. Upon his father's death in 1921, Alexander I went on to rule the country through an extended period of political crisis that culminated in the 6 January Dictatorship and, ultimately, his assassination in 1934. Prince Paul headed the state as a prince regent until Alexander's son Peter II was declared of-age, which happened following the Yugoslav coup d'état in March 1941. Alexander I was the longest reigning of the three Yugoslav monarchs.

Nikola I Petrović-Njegoš (Serbian Cyrillic: Никола I Петровић-Његош; 7 October [O.S. 25 September] 1841 – 1 March 1921) was the last monarch of Montenegro from 1860 to 1918, reigning as prince from 1860 to 1910 and as the country's first and only king from 1910 to 1918. His grandsons were kings Alexander I of Yugoslavia and Umberto II of Italy, among others.

Peter I (Serbo-Croatian: Петар I Карађорђевић, romanized: Petar I Karađorđević; 11 July [O.S. 29 June] 1844 – 16 August 1921) was King of Serbia from 15 June 1903 to 1 December 1918. On 1 December 1918, he became King of the Serbs, Croats and Slovenes, and he held that title until his death three years later. Since he was the king of Serbia during a period of great Serbian military success, he was remembered by Serbians as King Peter the Liberator and also as the Old King.

Peter was the fifth child and third son of Alexander Karađorđević, Prince of Serbia, and his wife, Persida Nenadović. Prince Alexander was forced to abdicate in 1858, and Peter lived with his family in exile. He fought with the French Foreign Legion in the Franco-Prussian War. He joined as a volunteer under the alias Peter Mrkonjić (Serbian Cyrillic: Петар Мркоњић, romanized: Petar Mrkonjić) in the Herzegovina uprising (1875–1877) against the Ottoman Empire. In 1883, Prince Peter married Princess Ljubica, daughter of King Nicholas I of Montenegro. Ljubica became known as Princess Zorka upon her marriage. Peter and Zorka had five children: Helen, Milena, George, Alexander, and Andrew. After his father died in 1885, Peter became head of the Karađorđević dynasty.

The 6 January Dictatorship (Serbian: Шестојануарска диктатура, Šestojanuarska diktatura; Croatian: Šestosiječanjska diktatura; Slovene: Šestojanuarska diktatura) was a royal dictatorship established in the Kingdom of Serbs, Croats and Slovenes (Kingdom of Yugoslavia after 1929) by King Alexander I (r. 1921–34) with the ultimate goal to create a Yugoslav ideology and a single Yugoslav nation. It began on 6 January 1929, when the king prorogued parliament and assumed control of the state, and ended with the 1931 Yugoslav Constitution.

Prince Paul of Yugoslavia, also known as Paul Karađorđević (Serbo-Croatian: Павле Карађорђевић, romanized: Pavle Karađorđević, English transliteration: Paul Karageorgevich; 27 April 1893 – 14 September 1976), was prince regent of the Kingdom of Yugoslavia during the minority of King Peter II. Paul was a first cousin of Peter's father, Alexander I.

Peter II Karađorđević (Serbo-Croatian: Петар II Карађорђевић, romanized: Petar II Karađorđević; 6 September 1923 – 3 November 1970) was the last King of Yugoslavia, reigning from October 1934 until he was deposed in November 1945. He was the last reigning member of the Karađorđević dynasty.

The eldest child of King Alexander I and Maria of Romania, Peter acceded to the Yugoslav throne in 1934 at the age of 11 after his father was assassinated during a state visit to France. A regency was set up under his cousin Prince Paul. After Paul declared Yugoslavia's accession to the Tripartite Pact in late March 1941, a pro-British coup d'état deposed the regent and declared Peter of age.