He was the son and namesake of the powerful Armenian Exarch of Africa, who had been one of East Roman Emperor Maurice's key generals in the 590 war with Persia. Though the younger Heraclius' birthplace is unknown, he grew up in Roman Africa; according to one tradition, he engaged in gladiatorial combat with lions as a youth.
In 608, the Heraclius the Elder renounced his loyalty to the Emperor Phocas, who had overthrown Maurice six years earlier. The rebels issued coins showing both Heraclii dressed as consuls, though neither of them explicitly claimed the imperial title at this time. The younger Heraclius' cousin Nicetas launched an overland invasion of Egypt; by 609, he had defeated Phocas's general Bonosus and secured the province. Meanwhile, the younger Heraclius sailed eastward with another force via Sicily and Cyprus. As he approached Constantinople, he made contact with leading aristocrats in the city, and soon arranged a ceremony where he was crowned and acclaimed as emperor. When he reached the capital, the Excubitors, an elite imperial guard unit led by Phocas's own son-in-law Priscus, deserted to Heraclius, and he entered the city without serious resistance. Heraclius personally executed Phocas. On October 5, 610, Heraclius was crowned for a second time, this time 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. Phocas's initial revolt had stripped the Danube frontier of troops, leaving the most of the Balkans at the mercy of the Avars. The Persian King Khosrau II, who had been an ally of Maurice, used his death as an excuse to launch a war against the Byzantines. Khosrau had at his court a man who claimed to be Maurice's son Theodosius, and Khosrau demanded that the Byzantines accept him as Emperor. The Persians had slowly gained the upper hand in Mesopotamia over the course of Phocas's reign; when Heraclius' revolt resulted in civil war, the Persians took advantage of the internal conflict to advance deep into Syria.
Heraclius offered peace terms to the Persians upon his accession, but Khosrau refused to treat with him, viewing him as just another usurper of Theodosius' throne. Heraclius' initial military moves against the Persians ended disastrously, and the Persians rapidly advanced westward. They took Damascus in 613, Jerusalem in 614 (damaging the Church of the Holy Sepulchre and capturing the Holy Cross in the process), and Egypt in 616. They made raids deep into Anatolia as far as Chalcedon and made contact and attempts to coordinate with the Avars.
The situation was so grave that Heraclius reportedly considered moving the capital from Constantinople to Carthage. But he remained in the East and worked on reorganizing the Byzantine military. 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.
Once he had rebuilt the army, Heraclius took the field himself in 621, the first emperor to campaign against a foreign enemy in person since Theodosius I. Confident that Constantinople was well defended, and unwilling to engage in a war of attrition over the lost eastern provinces, he marched across Asia Minor and invaded Persia itself. He would stay on campaign for several years. In 626, Constantinople itself was besieged by the Avars; but Persian attempts to cross the Bosporous and aid the Avars were repulsed by the Byzantine navy, and the Avars withdrew. Meanwhile, Heraclius acquired the assistance of the Khazars and other Turkic troops. Heraclius also exploited divisions within the Persian Empire, keeping the great Persian general Shahrbaraz neutral by convincing him that Khusrau had grown jealous of him and ordered his execution. At the Battle of Nineveh in 627, the joint Khazar-Byzantine force visited a definitive but not overwhelming defeat onto the Persian forces of Khosrau. When Khosrau still refused to make peace, Heraclius continued his campaign; as he approached the Persian capital of Ctesiphon, the Persian aristocracy deposed Khosrau. His successor made peace with Heraclius by restoring all the empire's former territories. The Persian Sassanid dynasty never recovered from this war; it took years for a strong king to emerge from a series of coups, and soon the Arabs overwhelmed the sinking state.
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 empire, and unknown to him at the time, Muhammad 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. Most of the inhabitants of these provinces were Monophysites who rejected the Council of Chalcedon. Heraclius tried to promote a compromise doctrine called Monothelitism; however, this philosophy was rejected as heretical by both sides of the dispute. For this reason, Heraclius was viewed as a heretic and bad ruler by some later religious writers. After the Monophysite provinces were finally lost to the Muslims, Monotheletism rather lost its raison d'etre and was eventually abandoned.