These two brothers had sought a reunion of the Eastern Church with the West and had endorsed the offer given by the popes and the Council of Basle-Ferrara-Florence, as had the Patriarch of Constantinople.
But Archbishop Mark of Ephesus and the majority of the Greek imperial officials blocked the imperial proposals and stuck to the Greek schism that had started in 1054.
This fatally weakened the Byzantine Empire and gave the Turkish Sultan his chance to take Constantinople which he did in 1453.
Nevertheless, despite all, the Emperor Constantine XI, last of the Paleologi emperors, did not flee but stayed with his soldiers to defend the city, the last great Roman imperial capital.
And he died, fighting the Infidel, at the head of his Greek imperial army, faithful to God, the Church, the 1500-year Greco-Roman Christian Empire.
Truly this was a great emperor, among the greatest of all time!
O Holy saints of the Byzantine Empire, pray for us!