However, in the 600's, when Arabs seized Egypt, Palestine, and Syria, the glory of the East Roman Empire began to fade. By the early 1200's, the Seljuk Turks had seized nearly all of Anatolia, while Serb, Bulgar, and other Balkan princes had broken free of Roman rule. When the Ottomans replaced the Seljuks in the late 1200's, they embarked on a slow but steady campaign to capture all Roman lands, present and former, in Anatolia and the Balkans and, eventually, to capture Constantinople itself.

Meanwhile, by 1365, the East Roman "Empire" had shriveled to only the capital and its immediate suburbs. The emperor retained his autonomy chiefly because the Ottomans needed time to consolidate their holdings elsewhere. But once the consolidation was accomplished, the empire's turn would come. The emperor, and everyone in Christendom, knew it.