Heraclius
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.