Construction had begun in 308 on the Basilica of Maxentius and Constantine (sometimes known as the Basilica Nova 'new basilica' or Basilica Maxentius).

Completed in 312 by Constantine I after his defeat of Maxentius at the Battle of the Milvian Bridge, it is the largest building in the Roman Forum.

A marvel of Roman engineering work, it is at the time of construction the largest structure to be built and thus is a unique building taking both aspects from Roman baths as well as typical Roman basilicas.

At this time, it uses the most advanced engineering techniques known including innovations taken from the Markets of Trajan and the Baths of Diocletian.

Similar to many basilicas at the time such as the Basilica Ulpia, the Basilica Maxentius features a huge open space in the central nave, but unlike other basilicas instead of having columns support the ceiling the entire building has been built using arches, a much more common appearance in Roman baths than basilicas.

Another difference from traditional basilicas is the roof of the structure.

While traditional basilicas were built with a flat roof, the Basilica Maxentius is built with a folded roof, decreasing the overall weight of the structure and decreasing the horizontal forces exerted on the outer arches.

Maxentius recovers Africa in 311, but in the following year must face the invasion of Italy by his brother-in-law Constantine at the head of a forty thousand-man army.

Constantine, scoring victories at Milan and Turin against superior numbers, wins two more victories at Brescia and Verona, and then marches south on Rome, reinforcing his army along the way.

Before the beginning of his final battle with Maxentius, Constantine reportedly sees a flaming cross in the sky.

Adopting the cross as a symbol for the fight, he supposedly vows to become a Christian if he emerges victorious.

On October 28, 312, Constantine’s forces, now about fifty thousand strong, defeat Maxentius and his seventy-five thousand-man army outside Rome at the Milvian Bridge over the Tiber River, in which Maxentius drowns while attempting to escape.

Constantine thus removes a dangerous rival and secures his share in the new government organized by Licinius.

Constantine's adherence to Christianity is closely associated with his rise to power.

He had fought the Battle of the Milvian Bridge in the name of the Christian God, having received instructions in a dream to paint the Christian monogram on his troops' shields.

This, at least, is the account given by the Christian apologist Lucius Caecilius Firmianus Lactantius; a somewhat different version, offered by Eusebius of Caesarea, tells of a vision seen by Constantine during the campaign against Maxentius, in which the Christian sign appeared in the sky with the legend “In this sign, conquer.” Despite the Emperor's own authority for the account, given late in life to Eusebius, it is in general more problematic than the other; but a religious experience on the march from Gaul is suggested also by a pagan orator, who in a speech of 310 referred to a vision of Apollo received by Constantine at a shrine in Gaul.

The southern Manchurian-based Korean kingdom Goguryeo extends its reach into the Liaodong peninsula, and Micheon of Goguryeo destroys the last Chinese prefecture, at Lelang, in 313, ending more than four hundred years of Chinese colonial presence.

The Three Kingdoms—Goguryeo, Baekje, and Silla—now dominate the Korean Peninsula.

Maximinus, angling for control of the entire Roman East, crosses the Bosporus with an army in 313 and attacks Licinius’ dominions in Thrace but, defeated at Tzurulum, east of Adrianople, is forced to retreat into Asia Minor, where he soon dies of disease.

Constantine goes far beyond the joint policy agreed upon at Mediolanum, however.

He has seemingly lived until now as a tolerant pagan, uncommitted to any one deity, but now begins gradually to adopt the Christian God as his protector.

By 313, he has already donated to the Bishop of Rome the imperial property of the Lateran, where a new cathedral, the Basilica Constantiniana (now S. Giovanni in Laterano), will soon rise.

The Church of St. Sebastian is also probably begun at this time.

Throughout his life, Constantine will ascribe his success to his conversion to Christianity and the support of the Christian God.

The triumphal arch erected in his honor at Rome after the defeat of Maxentius ascribes the victory to the “inspiration of the Divinity” as well as to Constantine's own genius.

A statue set up at the same time shows Constantine himself holding aloft a cross and the legend “By this saving sign I have delivered your city from the tyrant and restored liberty to the Senate and people of Rome.” The three-centuries long career of the privileged, politically influential Praetorian Guard ends when Constantine disbands it in 313.

A colossal head of Constantine, executed around 313, features blocklike facial planes and bulging eyes, whose off-center pupils gaze heavenward; the hair is a schematic cap.

Miltiades, pope from 311, had become the first pope after the edicts of toleration by the Roman emperors Galerius (ending the persecution of Christians), Maxentius (restoring church property to Miltiades), and Constantine (favoring Christianity).

Concurrently, however, dissension within the church has been caused by the Donatists, North African schismatics who contest the election of Caecilian as bishop of Carthage, championing the election and ordination of their bishop, the theologian Donatus.

At the Lateran Council of 313, Miltiades supports Caecilian and condemns the Donatists, who refuse to submit.

Constantine then orders the Council of Arles (Arelate), the first representative meeting of Christian bishops in the Western Roman Empire, but Miltiades dies before the council convenes.

(The modern Roman Catholic Church considers him a martyr because of earlier sufferings under the Roman emperor Maximian.)

Constantine, having conquered Italy and secured his regime as Emperor in 312, has begun promoting Christianity.

It is in these early years of Constantine’s reign that he begins issuing laws conveying upon the church and its clergy fiscal and legal privileges and immunities from civic burdens.

As he writes in a letter of 313 to the proconsul of Africa, secular offices should not distract the Christian clergy from their religious duties “… for when they are free to render supreme service to the Divinity, it is evident that they confer great benefit upon the affairs of state.” In another such letter, directed to the Bishop of Carthage, Constantine mentions the Spanish bishop Hosius.

Consecrated bishop of Córdoba around 295, Hosius had attended the Council of Elvira and from 312 has acted as Constantine’s ecclesiastical adviser.

Since he may well have been with Constantine in Gaul before the campaign against Maxentius, Hosius is possibly instrumental in the conversion of the Emperor to Christianity.

The Edict of Toleration, by which Constantine and his co-emperor Licinius officially recognize Christianity in 313, authorizes the toleration of different religions in the Roman Empire yet gives supremacy to Christianity.

Constantine annexes those portions of the empire ruled by the defeated Maxentius, then  meets Licinius at Mediolanum to confirm a number of political and dynastic arrangements.

A product of this meeting, the way to legalizing Christianity having been opened by Maxentius, will become known as the Edict of Milan, which extends toleration to the Christians and restores any personal and corporate property that had been confiscated during the persecution.

The extant copies of this decree are actually those posted by Licinius in the eastern parts of the empire.

Ancient Cirta, the prosperous capital of Numidia, destroyed during the civil war between Maxentius and the usurper Domitius Alexander (former governor of Africa), is renamed in 313 for Constantine, who rebuilds it.

Licinius adds the entire eastern half of the empire to his dominion.

In the same year, he marries Constantine's half sister Constantia.

During the campaign against Maximinus, Licinius has made his army use a monotheistic form of prayer closely resembling that will later be imposed by Constantine.

On June 5, 313, he issues an edict granting toleration to the Christians and restoring church property.

Hence, his contemporaries, Lactantius and Eusebius, hail him as a convert.

Around 305, after Diocletian began persecuting Christians, Lactantius had resigned his post as teacher of rhetoric at Nicomedia and begun a scholarly work on a systematic Latin summary of Christian teaching, Divinae institutiones (“Divine Institutions”), which he completes in about 313.

He now becomes tutor to Constantine’s son Crispus.

The Donatists, who establish their own communities, appoint bishops, and convene church councils, vigorously resist opposition from both the traditional church and the Roman state.

The rigoristic Donatist teachings proclaim that only the sinless can belong to the true church, that sacraments conferred by sinful ministers are invalid, and that only baptism conferred by a Donatist is valid.

Because the Donatists had denied the representative character of two earlier synods, at Rome and in Africa, at which they had been condemned, Constantine convenes the first representative meeting of Christian bishops in the Western Roman Empire at Arles in southern Gaul in August 314.

Attended by representatives of forty-three bishoprics, the Donatists are again condemned, but they reject the decisions reached by the council and again appeal to Constantine to review their case.