Although some Byzantine emperors occasionally attempted to re conquer some parts of the West, none were as successful as Justinian. The division between the two areas grew, resulting in a growing rivalry. While the Byzantine Empire continued after Justinian, the later emperors focused mainly on defending its traditional territory. From the 7th century onwards, the Byzantine Empire no longer had the necessary military strength, spelling the end of any hope for re conquest of the old Roman Empire's territories.
The greatest of the emperors was Justinian I (reigned 527-565), who with his able wife Theodora prepared for the re-conquest by defeating the Persians on the eastern frontier and extirpating various heresies that had alienated the Roman Catholic church. He sponsored a compilation and re-codification of Roman law and built the magnificent Hagia Sophia cathedral. Justinian's re-conquests of North Africa and Italy were short-lived.
The later years of his reign were marred by renewed war with the Persians and incursions by Bulgar and Slavic tribes, which created severe shortages of manpower and revenue. The weakened empire, preoccupied with internal problems, grew less and less concerned with the West. Although its rulers continued to style themselves "Roman" long after the death of Justinian, the term "Byzantine" more accurately describes the very different medieval empire.
By the Middle Ages, Byzantine armies had both Western and Eastern influences, with good heavy infantry and archers with strong early period infantry units like the Varangian Guard, strong militia units, and missile cavalry, but lacks late period units, gun technology, and spearmen.
The chief Church official in Constantinople, the patriarch, was appointed by the emperor and could be dismissed by him. Although on certain occasions in Byzantine history the patriarch stood up to his imperial master, he could not long sustain his position without imperial support. Thus the Church in Constantinople had all the power of the state behind it, while the popes in Rome were usually able to act more independently.
The emperors thus constantly came into conflict with the pope, who claimed jurisdiction over the patriarch. In the eighth century a controversy broke out between the Byzantine emperor Leo III and the pope concerning the use of icons or images in church worship, the emperor taking the position that the icons were an aid only to superstition while the pope claimed that they heightened religious feeling.
When Leo and his successors forbade the use of images and ordered them to be broken, the pope was powerless to do more than protest; but the Iconoclastic Controversy embittered relations between Rome and Constantinople for a century, and undoubtedly contributed to the desire of the popes to be protected by the Franks rather than by the Byzantine emperors, who were in their own view little better than heretics.
The ensuing reign of Phocas (602-610) may be described as a disaster. Khosrow seized the opportunity offered him by the murder of his benefactor, Maurice, to initiate a war of revenge that led Persian armies into the Anatolian heartland. Subsidies again failed to restrain the barbarians north of the Danube; after 602 the frontier crumbled, not to be restored save at the cost of centuries of warfare.
Lacking a legitimate title, holding his crown only by right of conquest, Phocas found himself confronted by constant revolt and rebellion. To contemporaries, the coincidence of pestilence, endemic warfare, and social upheaval seemed to herald the coming of the Antichrist, the resurrection of the dead, and the end of the world. Its spirit was manifest in 630, when Heraclius triumphantly restored the True Cross to Jerusalem, whence the Persians had stolen it, and--even more--when Constantinople resisted the Avar-Persian assault of 626.