Heraclius

Heraclius (; c. 575 – 11 February 641), was Eastern Roman (Byzantine) emperor from 610 to 641. His rise to power began in 608, when he and his father, Heraclius the Elder, the exarch (governor) of Africa, led a revolt against Phocas, an unpopular emperor. Heraclius called himself "basileus" and no more "augustus" since 629: he changed the "eastern roman empire" (partially latinised) into the "bizantine empire" (fully greek).
Rule.
Heraclius's rule was marked by a number of military campaigns. The year Heraclius came to power, the empire was threatened on many sides. Heraclius immediately took charge of the Byzantine–Sasanian War of 602–628. The first fights of the campaign ended in defeat for the Byzantines; the Persian army fought their way to the Bosphorus but Constantinople was protected by unbreakable walls and a strong navy, and Heraclius was able to avoid total defeat. Soon after, he started changes to rebuild and strengthen the military. Heraclius drove the Persians out of Asia Minor and pushed deep into their territory, defeating them clearly in 627 at the Battle of Nineveh. The Persian king Khosrow II was overthrown and executed by his son Kavad II, who soon for a peace treaty, agreeing to withdraw from all occupied territory. This way diplomatic relations were restored.
Loss of control.
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.
Entering diplomatic relations.
Heraclius entered diplomatic relations with the Croats and Serbs in the Balkans. He tried to repair the schism in the Christian church in regard to the Monophysites, by promoting a compromise doctrine called Monothelitism. The Church of the East (commonly called Nestorian) was also involved in the process. Eventually this project of togetherness was rejected by all sides of the argument.