Alexander I (Александар I Карађорђевић, aleksǎːndar př̩ʋiː karad͡ʑǒːrd͡ʑeʋit͡ɕ; – 9 October 1934), also known as Alexander the Unifier, 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 any monarch of the Kingdom of Yugoslavia.
Born in Cetinje, Montenegro, Alexander was the second son of Peter and Zorka Karađorđević. The Karađorđević dynasty 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 Ottomans 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.
In 1918, Alexander oversaw the unification of Serbia and the former Austrian provinces of Bosnia, Croatia and Slovenia into the Kingdom of Serbs, Croats and Slovenes. He ascended to the throne upon his father's death in 1921. An extended period of political crisis followed, culminating in the assassination of Croat leader Stjepan Radić. In response, Alexander abrogated the Vidovdan Constitution in 1929, prorogued the parliament, changed the name of the country to Kingdom of Yugoslavia and established a royal dictatorship. The 1931 Constitution formalised Alexander's personal rule and confirmed Yugoslavia's status as a unitary state, further aggravating the non-Serb population. Policital and economic tensions escalated on the outbreak of the Great Depression, which devastated the predominantly rural country.