When his brother Emperor John VIII Palaeologus died, a dispute erupted between Constantine and his brother Demetrius over the throne. They appealed to the Ottoman sultan Murad II to arbitrate the disagreement. He chose Constantine, who was crowned in Mistra on January 6, 1449. He attempted to marry a distant cousin, the widow of Murad II, but the courtship failed. Soon afterwards, Sultan Mehmed II began agitating for ownership of Constantinople. Desperate for any type of military assistance, Constantine appealed to the West, but he was refused help unless he united the Orthodox Church with the Roman Catholic Church, which was a policy pursued by his predecessors. He declared the churches united in 1452, but the union was overwhelmingly rejected by his subjects and it dangerously estranged him from his chief minister and military commander Megas Doux (Grand Duke) Lucas Notaras.
Mehmed II offered Constantine the chance to rule unmolested in Mistra before the siege of Constantinople, but he refused, preferring to fight and die defending his Empire. His wish would come true, as he was killed while defending the walls of Constantinople on May 29, 1453 (see Fall of Constantinople). His head was severed and displayed for days on a pike before he was buried with full honors in the city. Some Orthodox Christians consider him a saint but he has not been officially recognized as such. Thus the Roman Empire came to an end, whose origins could be traced back to the founding of Rome in 753 BC.