George, Crown Prince of Serbia in the context of King Alexander I of Yugoslavia

Alexander I Karađorđević (Serbo-Croatian: Александар I Карађорђевић, romanized: Aleksandar I Karađorđević, pronounced [aleksǎːndar př̩viː karadʑǒːrdʑevitɕ]; 16 December 1888 [O.S. 4 December] – 9 October 1934), also known as Alexander the Unifier (Aleksandar Ujedinitelj / Александар Ујединитељ [aleksǎːndar ujedǐniteʎ]), was King of the Serbs, Croats and Slovenes from 16 August 1921 to 3 October 1929 and King of Yugoslavia from 3 October 1929 until his assassination in 1934. His reign of 13 years is the longest of the three monarchs of the Kingdom of Yugoslavia.

Born in Cetinje, Montenegro, Alexander was the second son of Peter and Zorka Karađorđević. The House of Karađorđević had been removed from power in Serbia 30 years prior, and Alexander spent his early life in exile with his father in Montenegro and then Switzerland. Afterwards he moved to Russia and enrolled in the imperial Page Corps. Following a coup d'état and the murder of King Alexander I Obrenović in 1903, his father became King of Serbia. In 1909, Alexander's elder brother, George, renounced his claim to the throne, making Alexander heir apparent. Alexander distinguished himself as a commander during the Balkan Wars, leading the Serbian army to victory over the Ottoman Turks and the Bulgarians. In 1914, he became prince regent of Serbia. During the First World War, he held nominal command of the Royal Serbian Army.

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.

Princess Helen of Serbia (4 November [O.S. 23 October] 1884 – 16 October 1962) was a Serbian princess. She was the daughter of King Peter I of Serbia and his wife, the former Princess Ljubica of Montenegro. She was the elder sister of George, Crown Prince of Serbia and King Alexander I of Yugoslavia. Helen was also a niece of Queen Elena of Italy, Princess Anastasia of Montenegro (or "Stana"), wife of Grand Duke Nicholas Nikolaevich of Russia and of Princess Milica of Montenegro, wife of Grand Duke Peter Nikolaevich of Russia, the women who introduced Grigori Rasputin to Tsarina Alexandra.