Constantine the Great and the Beginning of Byzantium
It is a matter of debate when the Roman Empire officially ended and transformed into the Byzantine Empire. Most scholars accept that it did not happen at one time, but that it was a slow process, and so late Roman history overlaps with early Byzantine history. Constantine I ("the Great") is usually held to be the founder of the Byzantine Empire. He was responsible for several major changes that would help create a Byzantine culture distinct from the Roman past.
As emperor, Constantine enacted many administrative, financial, social, and military reforms to strengthen the empire. The government was restructured and civil and military authority separated. A new gold coin, the solidus, was introduced to combat inflation. It would become the standard for Byzantine and European currencies for more than a thousand years. The first Roman emperor to claim conversion to Christianity, Constantine played an influential role in the development of Christianity as the religion of the empire. In military matters, the Roman army was reorganised to consist of mobile field units and garrison soldiers capable of countering internal threats and barbarian invasions. Constantine pursued successful campaigns against the tribes on the Roman frontiers—the Franks, the Alamanni, the Goths, and the Sarmatians—even resettling territories abandoned by his predecessors during the turmoil of the previous century.
The age of Constantine marked a distinct epoch in the history of the Roman Empire. He built a new imperial residence at Byzantium and renamed the city Constantinople after himself (the laudatory epithet of "New Rome" came later, and was never an official title). It would later become the capital of the Empire for over one thousand years; for which reason the later Eastern Empire would come to be known as the Byzantine Empire. His more immediate political legacy was that, in leaving the empire to his sons, he replaced Diocletian's tetrarchy (government where power is divided among four individuals) with the principle of dynastic succession. His reputation flourished during the lifetime of his children and centuries after his reign. The medieval church upheld him as a paragon of virtue while secular rulers invoked him as a prototype, a point of reference, and the symbol of imperial legitimacy and identity.
Constantine the Great
Byzantine Emperor Constantine the Great presents a representation of the city of Constantinople as tribute to an enthroned Mary and Christ Child in this church mosaic. St Sophia, c. 1000 CE.
Constantinople and Civil Reform
Constantine moved the seat of the Empire, and introduced important changes into its civil and religious constitution. In 330, he founded Constantinople as a second Rome on the site of Byzantium, which was well-positioned astride the trade routes between East and West; it was a superb base from which to guard the Danube river, and was reasonably close to the Eastern frontiers. Constantine also began the building of the great fortified walls, which were expanded and rebuilt in subsequent ages. J. B. Bury asserts that "the foundation of Constantinople [...] inaugurated a permanent division between the Eastern and Western, the Greek and the Latin, halves of the Empire—a division to which events had already pointed—and affected decisively the whole subsequent history of Europe."
Constantine built upon the administrative reforms introduced by Diocletian. He stabilized the coinage (the gold solidus that he introduced became a highly prized and stable currency), and made changes to the structure of the army. Under Constantine, the Empire had recovered much of its military strength and enjoyed a period of stability and prosperity. He also reconquered southern parts of Dacia, after defeating the Visigoths in 332, and he was planning a campaign against Sassanid Persia as well. To divide administrative responsibilities, Constantine replaced the single praetorian prefect, who had traditionally exercised both military and civil functions, with regional prefects enjoying civil authority alone. In the course of the 4th century, four great sections emerged from these Constantinian beginnings, and the practice of separating civil from military authority persisted until the 7th century.
Constantine and Christianity
Constantine was the first emperor to stop Christian persecutions and to legalise Christianity along with all other religions and cults in the Roman Empire.
In February 313, Constantine met with Licinius in Milan, where they developed the Edict of Milan. The edict stated that Christians should be allowed to follow the faith without oppression. This removed penalties for professing Christianity, under which many had been martyred previously, and returned confiscated Church property. The edict protected from religious persecution not only Christians but all religions, allowing anyone to worship whichever deity they chose.
Scholars debate whether Constantine adopted his mother St. Helena's Christianity in his youth, or whether he adopted it gradually over the course of his life. According to Christian writers, Constantine was over 40 when he finally declared himself a Christian, writing to Christians to make clear that he believed he owed his successes to the protection of the Christian High God alone. Throughout his rule, Constantine supported the Church financially, built basilicas, granted privileges to clergy (e.g. exemption from certain taxes), promoted Christians to high office, and returned property confiscated during the Diocletianic persecution. His most famous building projects include the Church of the Holy Sepulchre, and Old Saint Peter's Basilica.
The reign of Constantine established a precedent for the position of the emperor as having great influence and ultimate regulatory authority within the religious discussions involving the early Christian councils of that time, e.g., most notably the dispute over Arianism, and the nature of God. Constantine himself disliked the risks to societal stability that religious disputes and controversies brought with them, preferring where possible to establish an orthodoxy. One way in which Constantine used his influence over the early Church councils was to seek to establish a consensus over the oft debated and argued issue over the nature of God. In 325 he summoned the Council of Nicaea, effectively the first Ecumenical Council. The Council of Nicaea is most known for its dealing with Arianism and for instituting the Nicene Creed, which is still used today by Christians.
The Fall of the Western Roman Empire
After Constantine, few emperors ruled the entire Roman Empire. It was too big and was under attack from too many directions. Usually, there was an emperor of the Western Roman Empire ruling from Italy or Gaul and an emperor of the Eastern Roman Empire ruling from Constantinople. While the Western Empire was overrun by Germanic barbarians—its lands in Italy were conquered by the Ostrogoths, Spain was conquered by the Visigoths, North Africa was conquered by the Vandals, and Gaul was conquered by the Franks—the Eastern Empire thrived. Constantinople became the largest city in the empire and a major commercial center. In 476 CE, the last Western Roman Emperor was deposed and the Western Roman Empire was no more. Thus the Eastern Roman Empire was the only Roman Empire left standing.