Constantine XI (1405-1453) was the last Byzantine emperor. A gallant prince, he completed the conquest of the Peloponnesus from the Latins and heroically commanded the futile defense of Constantinople against the Turks.

The fourth son of Emperor Manuel II Palaeologus (reigned 1391-1425), Constantine was born on Feb. 8, 1405. Following the Palaeologan custom of apportioning territorial responsibilities to each member of the reigning family, as a young man Constantine was assigned authority in the Black Sea coastal towns. His eldest brother, who had always favored him, became Emperor John VIII in 1425.

In 1427 Manuel's second son, Despot Theodore II of the Morea, announced his decision to resign his power in this important Peloponnesian territory. The Emperor designated Constantine to take Theodore's place. When Constantine arrived, however, Theodore had changed his mind. It was then agreed that Constantine should renew Byzantine efforts to conquer the areas of the Peloponnesus still in Latin hands, thus making an enclave for himself. He attacked Glarentza and finally won the city in 1428 by marrying the ruler's niece. By 1430 Constantine had conquered Patras and thus controlled the northern Peloponnesus. Two years later his younger brother Thomas annexed the last segments of Achaea, thereby placing all of the Peloponnesus in Byzantine hands for the first time since the Fourth Crusade (1204).

While John VIII attended the Council of Ferrara-Florence from 1437 to 1440, Constantine served as regent in Constantinople. During the following years he presided over what was to be the final flowering of Byzantine unity and prosperity in the Peloponnesus. At John VIII's death at the end of 1448, Constantine succeeded to the imperial throne. He proceeded cautiously regarding the hated agreements for Church union with the Latins, which John had accepted at Florence in hopes of winning Latin aid but which he had never implemented. Finally, under pressure from Rome, Constantine allowed the union to be proclaimed in Hagia Sophia on Dec. 12, 1452. This act greatly antagonized the bulk of his subjects, while it actually won him little effective help from the Latin West.

With only token help from outside, Constantine had to face the empire's last agony, as the Turkish sultan Mohammed II launched his great siege against Constantinople in early April 1453. The Turks finally broke into the city on May 29, 1453. Constantine died bravely during the ensuing sack.