The most important early emperor of Byzantium was Justinian, who ruled from 527 to 565. Justinian was the last Roman emperor to speak Latin as his native tongue; afterwards, all emperors spoke Greek. He is remembered for being both an incredibly fervent Christian, a major military leader, the sponsor of some of the most beautiful and enduring Byzantine architecture in existence, and the husband of probably the most powerful empress in the history of the empire, a former actress and courtesan named Theodora.
Justinian created a tradition that was to last for all of Byzantine history: that of the emperor being both the spiritual leader of the Christian church and the secular ruler of the empire itself. By the time the western empire fell, the archbishops of Rome had begun their attempts to assert their authority over the church (they would not succeed even in the west for many centuries, however). Those claims were never accepted in the east, where it was the emperor who was responsible for laying down the final word on matters of religious doctrine. Justinian felt that it was his sacred duty as leader of the greatest Christian empire in the world to enforce religious uniformity among his subjects and to stamp out heresy. He called himself “beloved of Christ,” a title the later emperors would adopt as well. While he was never able to force all of his subjects to conform to Christian orthodoxy (especially in rural regions far from the capital city), he did launch a number of attacks and persecutory campaigns against heretical sects.
One aspect of Justinian’s focus on Christian purification was the destruction of the ancient traditions of paganism in Greece and the surrounding areas initiated by his Christian predecessors. The Olympics had already been shut down by the emperor Theodosius I back in 393 CE (he objected to their status as a pagan religious festival, not an athletic competition). Justinian insisted that all teachers and tutors convert to Christianity and renounce their teaching of the Greek classics; when they refused in 528, he shut down Plato’s Academy, functioning at that point for almost 1,000 years.
Justinian did not just enforce religious uniformity, he also imposed Roman law on all of his subject peoples. The empire had traditionally left local customs and laws alone so long as they did not interfere in the important business of tax collection, troop recruitment, and loyalty to the empire. Justinian saw Roman law as an aspect of Roman unity, however, and sought to stamp out other forms of law under his jurisdiction. He had legal experts go through the entire corpus of Roman law, weed out the contradictions, and figure out the laws that needed to be enforced. He codified this project in the Corpus Juris Civilis, which forms the direct textual antecedent for most of the legal systems still in use in Europe.
Theodora, who had come from decidedly humble origins as an entertainer, worked diligently both to free prostitutes from sexual slavery, expand the legal rights and protections of women, and protect children from infanticide. She was Justinian’s confidant and supporter throughout their lives together, helping to conceive of not just legal revisions, but the splendid new building projects they oversaw in Constantinople. In a famous episode from early in Justinian’s reign, Theodora prevented Justinian and his advisors from fleeing from a massive riot against his rule, instead inspiring Justinian to order a counter-attack that may well have saved his reign. While most political marriages in Byzantium, as in practically every pre-modern society, had nothing whatsoever to do with love or even attraction, Theodora and Justinian clearly shared both genuine affection for one another and intellectual kinship.
Justinian was intent on re-conquering the western empire from the Germanic kings that had taken over. He was equally interested in imposing Christian uniformity through the elimination of Christian heresies like Arianism. He sent a brilliant general, Belisarius, to Vandal-controlled North Africa in 533 with a fairly small force of soldiers and cavalry, and within a year Belisarius had soundly defeated the Vandal army and retaken North Africa for the empire. From there, Justinian dispatched Belisarius and his force to Italy to seize it from the Ostrogoths.
What followed was twenty years of war between the Byzantines and the Gothic kingdom of Italy. The Goths had won over the support of most Italians through fair rule and reasonable levels of taxation, and most Italians thus fought against the Byzantines, even though the latter represented the legitimate Roman government. In the end, the Byzantines succeeded in destroying the Gothic kingdom and retaking Italy, but the war both crippled the Italian economy and drained the Byzantine coffers. Italy was left devastated; it was the Byzantine invasion, not the “fall of Rome” earlier, that crippled the Italian economy until the late Middle Ages.
In 542, during the midst of the Italian campaign, a horrendous plague (the “Plague of Justinian”) killed off half the population of Constantinople and one-third of the empire's population as a whole. This had an obvious impact on military recruiting and morale. In the long term, the more important impact of the plague was in severing many of the trade ties between the two halves of the empire. Economies in the west became more localized and less connected to long-distance trade, which ultimately impoverished them. A few years earlier, in 536, a major volcanic eruption in Iceland spewed so much debris in the air that Europe’s climate cooled considerably with “years without a summer,” badly undermining the economy as well. Thus, war, natural disaster, and disease helped usher in the bleakest period of the Middle Ages in the west, as well as leading to a strong economic and cultural division between east and west.
Even as the Byzantine forces struggled to retake Italy, Justinian, like the emperors to follow him, had a huge problem on his eastern flank: the Persian Empire. Still ruled by the Sassanids, the Persians were sophisticated and well-organized opponents of the empire who had never been conquered by Rome. Ongoing wars with Persia represented the single greatest expense Justinian faced, even as he oversaw the campaigns in Italy. The Byzantines and Persians battled over Armenia, which was heavily populated, and Syria, which was very rich. Toward the end of his reign, Justinian simply made peace with the Persian king Khusro I by agreeing to pay an annual tribute of 30,000 gold coins a year. It was ultimately less expensive to spend huge sums of gold as bribes than it was to pay for the wars.
The problem with Justinian's wars, both the reconquest in the west and the ongoing battles with the Persians in the east, was that they were enormously expensive. Because his forces won enough battles to consistently loot, and because the empire was relatively stable and prosperous under his reign, he was able to sustain these efforts during his lifetime. After he died, however, Byzantium slowly re-lost its conquests in the west to another round of Germanic invasions, and the Persians pressed steadily on the eastern territories as well.