Excerpts from Books and Wikipedia
"In 634 the Muslims marched into Roman Syria, defeating Heraclius's brother Theodore. Within a short period of time, the Arabs conquered Mesopotamia, Armenia and Egypt. Heraclius entered diplomatic relations with the Croats and Serbs in the Balkans." Heraclius was Emperor of the Byzantine Empire from 610 to 641. [Wikipedia] The dates 477-641 coincide with the end of the Western Roman Empire (477) and the effective end of the Byzantine Empire (641).