From Wikipedia

Byzantium, The Lost Empire

John Romer

Episode Four

Forever And Ever

The decline of the Byzantine Empire was a process similar to the decline of the Western Roman Empire, in that it lasted many centuries. There is no clear consensus on when this process began; but many dates and time lines have been proposed by historians.

Map of the changes in borders of the Byzantine Empire. The dates represented are 476 (Fall of the Roman Empire; Basiliscus deposed and Zeno restored), 550 (Justinian I's western reclamations; Ostrogothic Kingdom), 717 (Leo III reign; 2nd Arab siege), 867 (Basil I reign begins), 1025 (Basil II dies; Constantine VIII reign begins), 1095 (Alexius I Comnenus requests western aid against the Seljuk Turks), 1170 (Amalric I and Manuel I alliance), 1270 (Michael VIII riegn), and 1400 (Closing of the Byzantine–Ottoman Wars).