Heraclius
However, Heraclius soon lost many of his newly regained lands to the Rashidun Caliphate. Newly appearing from the Arabian Peninsula, the Muslims quickly won the Sasanian Empire by force. In 636, 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 responded with changes which let his successors fight the Arabs and avoid total destruction.