Constantine the Great (ca. 272-337) - Constantine was the last significant emperor of what we traditionally think of as the Roman Empire. And it was during his reign and due mostly to his actions that the empire was transformed from old to new.  In fact, the steps that he took were responsible for extending the Roman Empire's life far beyond what it would have been without his reforms. Not that he had much choice; he simply examined events as they were happening and reacted in the best way possible to preserve it. The transformation occurred primarily due to two major changes during his reign. One, he moved the capital of the empire from Rome to the Greek city of Byzantium. Although it was not yet an official split of the kingdom into two, it was a first step. Diocletian had actually laid the foundation for the eventual split by his establishment of the tetrarchy in 293. The Roman Empire was unevenly divided in terms of its wealth. The eastern region had the lion's share. As the older half, it was much more developed than the west. Greece, Egypt, Asia Minor all had had civilizations stretching back centuries. The west, by comparison, was new and underdeveloped. Constantine realized that the empire would be more manageable from the east. In effect, he was abandoning the west. It was slowly deteriorating under Roman corruption and neglect, although the empire would still technically be united for several more years. Byzantium was ideally located for the new capital. Situated on the Bosphorus, it was surrounded by water on three sides making it easy to defend and highly conducive to trade by sea. Constanine renamed it "New Rome", but the Romans quickly referred to it as Constantinople in honor of of the emperor who made it the capital.

 

The other big change was his "conversion" to Christianity (I put conversion in parentheses because many Christian scholars question its authenticity). The conversion famously occurred in 312 at the Battle of Milvian Bridge. Constantine's transition to emperor was not a smooth one. Recall that Diocletian's own ascension was not without conflict. Recall also that, in an attempt to more efficiently govern the empire, Diocletian had appointed both Maximian and Constantius to help him rule (See Diocletian). Well Constantius was the father of Constantine, and Maximian was the father of a man named Maxentius. Upon retiring, Diocletian compelled Maximian to retire as well, with the expectation that Constantine and Maxentius would take over. And upon his own father's death in 306, Constantine assumed power. However, Galerius, the fourth member of the original tetrarchy, refused to recognize Maxentius' authority. The tetrarchy had divided the empire into regions. Constantine initially controlled the region "beyond the Alps", essentially Gaul and Britannia. Maxentius, who was resentful of Constantine, responded by seizing power in Rome and claiming Italy as his region. Like so many times in Rome's past, multiple claimants to the throne led to civil war. Galerius, who controlled the east, responded to Maxentius's move by sending an army to Italy to depose him. It failed, and Maxentius strengthened his hold on power. However, he was not popular with the Roman citizens. Initially, Constantine remained aloof and simply monitored events from the north. This had the dual effect of letting his rivals fight until only one was left, and raising his stature among the people by appearing to be uninterested in power.

 

After defeating Galerius' army, Maxentius stood alone, and Constantine decided to make his move. He marched his army south into Italy. Maxentius heard of his approach and prepared to counter Constantine's move. Initially, he planned to barricade himself within Rome's walls and force Constantine to siege it. However, he had become so unpopular in Rome that unrest began to grow. So he decided that an open confrontation was better. He was able to raise an army about twice the size of Constantine's and was confident of victory. Before the battle, Constantine reportedly received a sign which would guarantee his victory. The event is recorded differently in two sources. According to Lactantius, he had a dream the night before in which a voice told him to emblazon his army's shields with with the sign of Christ, which is known as a Chi Rho. However, Eusebius writes that while marching to battle, the sign appeared in the sky, with the message, In Hoc Signo Vinces (with this sign, you will conquer). And so, when the two armies met at Milvian Bridge, Maxentius' men noticed a symbol on the shields of the opposing army with which they were unfamiliar. Despite being outnumbered two to one, Constantine won the battle in short order. His cavalry broke the enemy's after the initial engagement. And his infantry pushed the opposing infantry back into the Tiber where they were hacked down or quickly drowned. Constantine credited Christ with his victory and was now sole ruler in the west. In the interim, Galerius died in 311 and a man named Lucinius seized power in the east. Initially, Constantine and Lucinius formed an uneasy alliance and jointly issued the Edict of Milan in 313, recognizing Christianity along with all existing religions throughout the empire. However, their alliance fell apart by 324 and the two were engaged in a second civil war. Constantine won a series of victories, culminating in the Battle of Chrysopolis. Constantine was now sole ruler of the Roman Empire which opened the way for the aforementioned transfer of the capital to Constantinople. He reigned until his death in 337.