From soldier to sole ruler of the Roman Empire, Constantine the Great transformed ancient history. His reign marked the rise of Christianity, the founding of Constantinople, and a lasting shift in imperial power from West to East.

Constantine the Great | Constantin le Grand | Konstantin der Große | Constantino el Grande | Konstantyn Wielki | 君士坦丁大帝 | 콘스탄티누스 대제 | コンスタンティヌス大帝

Over the past few years – roughly the same period during which I’ve been writing this blog – I’ve repeatedly come across traces of Emperor Constantine, often in places far apart from one another. Sometimes it happened by chance, other times more deliberately. The first encounter was in far-off York, in northern England. I was busy taking photos and probably wasn’t paying full attention to the guide, but the statue of Constantine didn’t surprise me. After all, who hasn’t heard of this emperor? He was the one who founded Constantinople – the eastern capital of the Roman Empire, named in his honour.

Later, in the Vatican, I visited the Room of Constantine in the Raphael’s Rooms, richly decorated with frescoes illustrating scenes from his life. A few years on, during another visit to Rome, I took close-up photos of the Arch of Constantine – by then already aware that my next journey would soon take me to Istanbul, once known as Constantinople, and home to the famous Hagia Sophia. Even though the great basilica has seen better days, it still houses remarkable Byzantine mosaics, including one of Constantine offering his city as a religious gift.

And so, the time has come for a post about the emperor and visionary – Constantine the Great. He was one of the most significant emperors in Roman history. Under his rule, Christianity ceased to be persecuted. He was not only a brilliant military strategist but also a far-sighted leader. It was Constantine who moved the capital of the empire to Byzantium, renaming it Constantinople – today’s Istanbul. Although he was only baptised towards the end of his life, his impact on the development of Christianity was profound. He also convened the first ecumenical council – the Council of Nicaea – which played a crucial role in shaping the doctrine of the Church.

Historical Context

By the end of the third century, the Roman Empire was plunged into chaos: civil wars, economic crises, and external threats had seriously weakened the state. In response, Emperor Diocletian introduced a new system of rule – the tetrarchy – a model of shared power involving four emperors: two Augusti (senior emperors) and two Caesares (their junior partners and designated successors). The system was designed to ensure more effective administration of the vast empire and prevent succession conflicts. In 293 AD, Diocletian established the original tetrarchy as follows: Diocletian – Augustus of the East; Maximian – Augustus of the West; Galerius – Caesar of the East; and Constantius Chlorus – Caesar of the West. The tetrarchy functioned relatively effectively for about 10 to 12 years (293–305), but the following decade was marked by escalating succession crises and power struggles.

In 305 AD, an unprecedented event occurred in Roman history – Diocletian and his co-ruler Maximian voluntarily abdicated, stepping down from imperial power. Diocletian, weakened by illness, retired from public life and withdrew to his palace in Spalatum (modern-day Split), aiming to set a precedent: that imperial authority should not be lifelong or hereditary, but rather serve the stability of the state and operate on a principle of orderly transition. Maximian also relinquished his office, though unlike Diocletian, he struggled to let go of power and would later attempt to return to politics.

Upon their abdication, the senior emperors did not promote their own sons but instead selected trusted associates as the new Caesars – Flavius Severus (Severus II) in the West and Maximinus Daza in the East – both loyal military officers, particularly to Galerius. This deliberate sidelining of imperial sons broke with traditional expectations of dynastic succession. Among those overlooked were Constantine, son of the newly appointed Augustus Constantius Chlorus, and Maxentius, son of the retired emperor Maximian. Although the tetrarchy was based on merit rather than bloodline, this decision sparked unrest. When Constantius died in 306, his troops in Britain unilaterally declared Constantine emperor. Not long after, Maxentius seized power in Rome. Over the next several years, the empire descended into civil wars as multiple claimants fought for control. The final struggle for sole rule played out between three rivals: Constantine, who commanded support in the western provinces; Maxentius, who held power in Italy and the capital, Rome; and Licinius, who governed the eastern territories. It was between these men that the future of the Roman Empire would ultimately be decided.

A Difficult Path to Power

Constantius Chlorus, the father of Constantine, embarked on a military campaign to Britain in 305 AD as the western Augustus, aiming to suppress Pictish raids and secure the northern frontier of the Roman Empire. This was a strategically significant operation: earlier barbarian attacks had unsettled the region, and the emperor sought to reaffirm his authority. At the time, Constantius was seriously ill. His son, Constantine, was then at the court of the eastern Augustus, Galerius – likely as a political hostage. Upon learning of his father’s condition and impending campaign, Constantine either escaped or was granted permission to join him. He travelled rapidly across Gaul to reunite with his father. Once in Britain, Constantine participated in the campaign against the Picts, though likely not on the front lines. His exact tactical role remains unclear, but sources confirm his presence alongside his father in military operations.

As Constantius’s condition deteriorated, he withdrew to Eburacum (modern-day York), where he died shortly thereafter. In response, the legions stationed there, impressed by Constantine’s leadership and involvement, proclaimed him emperor. This was a typical instance of military acclamation, with strong precedents in Roman history (e.g. Septimius Severus, Claudius). Although technically illegal – violating the tetrarchic succession system – the army viewed Constantine as the natural heir, being both a commander and the son of the deceased Augustus. Nevertheless, other claimants remained, and Constantine spent the following years consolidating power and defeating rivals.

Nearly two millennia later, to commemorate Constantine’s proclamation as emperor in York in AD 306, a bronze statue was unveiled in 1998 near York Minster. Commissioned by the York Civic Trust and created by sculptor Philip Jackson, the monument highlights the city’s Roman legacy and its role in a turning point of imperial and Christian history.

Statue of Constantine the Great in York. It was here, in the city of York, that Constantine was proclaimed emperor by the army in 306 AD after the death of his father.

Following the death of Constantius Chlorus, his son Constantine was proclaimed emperor by the legions. However, the eastern emperor Galerius recognised him only as Caesar, not Augustus. Constantine, nonetheless, began to consolidate his power in the western provinces, successfully defending the Rhine frontier, battling Germanic tribes, and securing the loyalty of the army, while avoiding direct conflict with other tetrarchs. In 307 AD, he formed an alliance with Maximian, the former Augustus and father of Maxentius, marrying Maximian’s daughter Fausta. However, relations with Maxentius, who ruled Italy, deteriorated in the following years.

After Galerius’s death in 311, Constantine declared himself Augustus, abandoning all pretence of adhering to the tetrarchic order. In 312, he launched a military campaign against Maxentius, culminating in a decisive victory at the Battle of the Milvian Bridge near Rome. According to tradition, on the eve of battle he saw a vision of the cross in the sky with the words In hoc signo vinces (In this sign, you will conquer), marking the beginning of his alignment with Christianity.

After taking control of Rome, Constantine solidified his rule over the western Roman Empire. In 313 AD, together with Licinius, co-emperor of the East, he issued the Edict of Milan, which granted religious freedom to Christians and brought an end to their persecution. However, the alliance with Licinius soon collapsed. In 316, a first military conflict broke out between them, culminating in Constantine’s victory at the Battle of Cibalae, which extended his influence over the Balkans. Yet, the final resolution of their rivalry would come only later. In 315, to mark the tenth anniversary of Constantine’s reign, the monumental Arch of Constantine was erected in Rome. It celebrated his triumph over Maxentius and symbolically affirmed his status as one of the empire’s leading rulers.

The Arch of Constantine in Rome, dedicated in 315 CE by the Senate and People of Rome, celebrates the tenth anniversary (decennalia) of Constantine’s reign and his general victories.

Constantine now faced a decisive confrontation with his final rival, Licinius, who ruled the eastern part of the empire. The conflict ended in 324 AD, when Constantine secured a decisive victory in battles at Adrianople, Chalcedon, and Chrysopolis (modern-day Üsküdar in Istanbul). This triumph made him the sole ruler of the Roman Empire – for the first time in several decades.

Master and Visionary

After seizing complete control of the empire, Constantine deliberately distanced himself from Rome, which he found politically and symbolically problematic. The city remained dominated by a pagan Senate, closely tied to the traditions of the old Republic. Though Constantine never converted during his lifetime, he increasingly aligned his rule with Christianity and sought a new centre of power that reflected the ideological and strategic realities of his reign.

From 324 AD, Constantine began spending more time in Asia Minor, particularly in Nicomedia, Diocletian’s former capital and a key eastern administrative hub. It was here that he established his imperial office and began preparing for the foundation of a new capital. That same year, he selected Byzantium – a historic Greek city on the Bosphorus – as the location for his new seat of power. Strategically placed between Europe and Asia, easily fortified, and with excellent maritime access, Byzantium offered both symbolic and logistical advantages. Today, Istanbul remains one of Europe’s largest cities, with a population three times that of greater Rome.

Meanwhile, in 325 AD, Constantine convened the First Ecumenical Council of the Christian Church in nearby Nicaea (modern İznik). Although unbaptised, he presided over the council and sought to enforce religious unity across the empire.

During this period, Constantine pressed ahead with the transformation of Byzantium into a grand new capital. From 326 AD, the city underwent a major reconstruction. The imperial palace was built alongside with a new forum, temples, baths, Christian basilicas, city walls, and harbours. The city was formally inaugurated on 11 May 330 AD as Nova Roma (New Rome). Yet it was almost immediately referred to as Constantinople – City of Constantine, a name that would endure for centuries. The new capital symbolised a break with Rome’s pagan past, the emergence of a Christian empire, and the rise of a centralised imperial monarchy. Constantine relocated part of the administration there, and in time, Constantinople would fully assume the role of the eastern imperial centre – later the heart of the Byzantine Empire.

A 10th-century mosaic at Hagia Sophia in Istanbul. Emperor Constantine offers the city of Constantinople to the Virgin Mary. It reflects the imperial theology of Byzantium – where rulers were seen as sacred patrons of the Christian world.

After founding his new capital, Constantine continued to strengthen his rule and pursue his vision of a reformed empire. Constantinople quickly rose to prominence as the chief imperial centre – the emperor relocated the court, officials, and part of the army there, making the city not only a symbolic seat of power, but also a functional hub for managing the increasingly important eastern provinces. As Constantine resided less frequently in Rome, Constantinople emerged as its full-fledged successor.

At the same time, Christianity, though not yet the official state religion, was becoming an integral part of imperial identity. Constantine supported the Church, funding major basilicas – including the Church of the Holy Sepulchre in Jerusalem and St Peter’s Basilica in Rome – and granting the clergy privileges and property. Though unbaptised for most of his life, his policies consistently favoured Christians.

After 330 AD, Constantine focused increasingly on consolidating the changes he had introduced. He completed Diocletian’s administrative reforms, restructured the army, centralised power, and began to portray imperial authority in quasi-sacred terms. The emperor came to be seen not only as a ruler, but as a “divinely appointed” leader, protector of the Church and supreme religious authority. His ideological programme rested on uniting imperial power with Christianity, while rejecting the pagan institutions of the old order.

In the final years of his life, Constantine prepared for succession. His three sons – Constantine II, Constans, and Constantius II – were granted the title of Caesar and designated to divide the empire between them. In 337 AD, while preparing for a planned campaign against the Persians, Constantine fell ill near Nicomedia, where he received baptism. It was a symbolic yet belated act – although a lifelong supporter of the Church, he only formally became a Christian on his deathbed.

He died on 22 May 337 AD, and his body was transported to Constantinople, where he was buried in the Church of the Holy Apostles – a site he had chosen as the imperial mausoleum. The church, though rebuilt several times, survived for centuries. However, during the Fourth Crusade in 1204, when crusaders captured and plundered Constantinople, it was destroyed and looted. Constantine’s sarcophagus was broken, and his remains were likely desecrated or lost.

Remains of the Hippodrome of Constantinople at Sultanahmet Square, Istanbul. This open space in modern Istanbul was once the heart of Byzantine Constantinople – the Hippodrome, expanded by Constantine in the 4th century CE.

After Constantine’s death, his three sons assumed power, dividing the empire among themselves. Soon after, a wave of purges swept through the imperial family, resulting in the murder of many of Constantine’s relatives. Although the empire would remain formally united for decades, its spiritual and political centre had permanently shifted eastwards—to the city that would bear Constantine’s name for the next thousand years.