He was the son of the powerful Exarch of Carthage and had been one of East Roman Emperor Maurice's key generals in the 590 war with Persia. With the support of Priscus, one of Emperor Phocas' top military leaders, the patriarch Sergius and the Green political faction, Heraclius overthrew his predecessor and personally executed him. On October 5, 610, Heraclius was crowned in the Chapel of St. Stephen within the Great Palace and at the same time wed his betrothed, Fabia, who took the name Eudocia. She was beloved in Constantinople, and when she died in 612 and he married his niece Martina, the second marriage was never approved of. In the reign of Heraclius' two sons, the divisive Martina was to become the center of power and political intrigue.
When Heraclius took power, the Empire was in a desperate situation and he considered moving the capital from Constantinople to Carthage. He developed the idea of granting land to individuals in return for hereditary military service. The land so granted was organised into thema, a Greek word to describe a division of troops, and each theme was placed under the command of a strategos or military governor. This arrangement ensured the continuance of the Empire for hundreds of years and enabled Heraclius to reconquer lands taken by the Persians, ravaging Persia along the way.
During the reign of Phocas and the first decade under Heraclius, the empire had crumbled, losing control of Egypt, Syria, Armenia and Palestine to the Persians and most of the Balkans to the Avars. The Persians under King Khosrau II took Damascus in 613, and Jerusalem in 614, when they destroyed the Church of the Holy Sepulchre and captured the Holy Cross. Twice, the city of Constantinople itself fell under siege, once by both the Persians and Avars working in concert.
Heraclius responded to this desperate situation by taking the field himself in 621. Confident that Constantinople was well defended, he marched across Asia Minor and invaded Persia. Along the way, he acquired the assistance of Khazarian and Turkic troops. At the Battle of Nineveh in 627, the joint Khazar-Byzantine force routed the Persian forces of Khosrau. When Khosrau still refused to make peace, Heraclius continued his campaign, taking the Persian capital of Ctesiphon. Khosrau was deposed, and his successor made peace with Heraclius by restoring all the empire's former territories. The Persian Sassanid dynasty, however never recovered from this war, and soon passed into history.
Heraclius took for himself the ancient Persian title of "King of Kings", dropping the traditional Roman imperial title of "Augustus". Later on, he styled himself as Basileus, the Greek word for "Emperor", and that title was used by the eastern Roman emperors for the next 800 years. Heraclius also discontinued the use of Latin as the empire's official language, replacing it with Greek. Although the empire called itself Roman throughout the rest of its history, it was in reality a Hellenic empire from Heraclius onward.
In 630, he reached the height of his power when he marched triumphantly into Jerusalem and restored the True Cross to the Church of the Holy Sepulchre. But unfortunately for his war-weary nation, and unknown to him at the time, Mohammed had only recently succeeded in unifying all the nomadic tribes of the Arabian Peninsula. The Arabs, who had been too divided in the past to pose a military threat, now comprised one of the most powerful states in the region, and were animated by their new conversion to Muhammad's religion of Islam.
Heraclius fell ill soon after his triumph and never took the field again. When the Arab Muslims invaded Syria and Palestine in 634, he was unable to oppose them personally, and his generals failed him. The Battle of Yarmuk in 636 resulted in a crushing defeat for the larger Roman army and within three years, Syria and Palestine were lost again. By the time of Heraclius' death, most of Egypt had fallen as well.
Although his defeat of the Persians produced no lasting benefit to the empire, Heraclius still ranks among the greatest of the Byzantine emperors. His reforms of the government reduced the corruption which had taken hold in the disastrous reign of Phocas, and he reorganized the military with great success. Ultimately, the reformed imperial army halted the Muslims in Asia Minor and held on to Carthage for another 60 years, saving a core from which the empire's strength could be rebuilt.
The recovery of the eastern areas of the Byzantine Empire from the Persians once again raised the problem of religious unity, centering around the understanding of the true nature of Christ (see Monophysites).