What did Constantine XI Palaiologos do?

Constantine XI Palaiologos was the last emperor of the Byzantine Empire. He began his political career as the ruler of the southern Byzantine principality of Morea. At that time, the Byzantine Empire was just a shadow of its former self; the Ottomans had conquered most of the Byzantine territory. During the confrontation between the Ottoman ruler Murad II and the tiny principality of Morea, the Ottoman army broke through Constantine’s lines of defense and defeated him.
When Constantine became an emperor in 1449, he faced an Ottoman invasion; the new sultan, Mehmet II, wanted to capture the imperial capital Constantinople. In order to defend the capital, Constantine needed military help from the west. Accordingly he agreed to a union between the Catholic and the Orthodox churches and accepted the supremacy of the Roman pope. That concession did not go over well with many of his Orthodox subjects, some of whom considered Catholicism a heresy and preferred Ottoman domination to surrender to Roman demands. Constantine risked their displeasure in the hope of substantial military aid the west.
As it turned out, Constantine’s hopes were not justified. Only a small number of Italian soldiers and merchants participated in the defense of Constantinople during the last siege in 1453. Mehmet urged Constantine to capitulate and promised in return to spare Constantine’s life and allow him to continue to rule in one small part of his empire; instead, Constantine declined and led the defensive effort of his dwindling army and the people of Constantinople. Despite intensive attacks and artillery bombardments, they managed to resist for several months .
When the enemy army finally broke through the city walls, Constantine took off his imperial regalia and died fighting as a common soldier alongside his troops on May 29, 1453. His courageous leadership made him into a Greek national hero. A statue of him stands in Athens.