Princess Helen of Serbia in the context of Princess Anastasia of Montenegro

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.