Constantine was proclaimed emperor by his soldiers in Eboracum (York, UK) on the 25th of July AD 306. Constantine’s father was part of Diocletian’s Tetrarchy and this method of rule quickly fell apart after Diocletian and Maximian retired as Augusti of the East and West in AD 305. By 308 there were multiple claimants to the role of Augustus and only one that was to be a Caesar (next in line to be emperor). What followed was a series of civil wars until 313 when it was just Constantine and Licinius left. Constantine finally defeated Licinius and declared himself sole emperor in 324, reuniting the empire.
During these civil wars Constantine forced Maximian to commit suicide in 310 and then defeated his son Maxentius at the Battle of the Milvian Bridge in 312. According to Lactantius, Maxentius had consulted the Sibylline Books and was told that an enemy of Rome would perish.
We are told that the night before the battle Constantine received a vision which led to him fight under the Chi-Rho symbol. Eusebius gives two versions of the battle. The first in Ecclesiastical History doesn’t mention the vision, just promotes the idea that the Christian God helped. The second account in Life of Constantine mentions the vision, going into great detail, including the use of the phrase "Εν Τούτῳ Νίκα", En toutō níka” translating to “in this sign, conquer”.
Despite founding Constantinople to serve as his new capital, Constantine did create several buildings in Rome and repurposed others. The most famous examples are the Arch of Constantine, the Basilica of Maxentius (below, off center, as seen from the Palatine Hill) in the Forum Romanum and the Lateran complex.
The Arch of Constantine was dedicated on July 25th AD 315 near the Colosseum, spanning the Via triumphalis and uses several decorative pieces from monuments of the earlier emperors; Trajan, Hadrian, and Marcus Aurelius. The re-working of decoration of previous emperors’ monuments includes the remodelling of their faces (if shown) to look more like Constantine. The roundels are from the time of Hadrian, the statues of captured Dacians are from Trajan and the four panels on each side of the inscription panel are from a monument of Marcus Aurelius. The inscription itself seems to suggest at Constantine’s changing religious beliefs, especially the term ‘inspired by the divine’ (instinctu divinitatis).
imp(eratori) · caes(ari) · fl(avio) · constantino · maximo · p(io) · f(elici) · avgusto · s(enatus) · p(opulus) · q(ue) · r(omanus) · qvod · instinctv · divinitatis · mentis · magnitvdine · cvm · exercitv · svo · tam · de · tyranno · qvam · de · omni · eivs · factione · vno · tempore · ivstis · rempvblicam · vltvs · est · armis · arcvm · trivmphis · insignem · dicavit
To the Emperor Caesar Flavius Constantinus, the greatest, pious, and blessed Augustus: because he, inspired by the divine, and by the greatness of his mind, has delivered the state from the tyrant and all of his followers at the same time, with his army and just force of arms, the Senate and People of Rome have dedicated this arch, decorated with triumphs.
More about Rome's arches via our Ancient Rome Live project
The Basilica of Maxentius was begun under Maxentius in 308 and completed by Constantine after his defeat of Maxentius in 312. It was then renamed the Basilica Nova. The Colossus of Constantine (below), parts of which can be seen in the Capitoline Museums was erected in the Basilica. While the general multi-purpose use of the Basilica did not change, the addition of the colossus and the renaming of it portrayed the decisive victory of Constantine over his rival, also possibly an attempt to unofficially damnatio Maxentius.
The Lateran Basilica stands on top of the site of the ‘New Fort of the Roman imperial cavalry bodyguards’, which was built under Septimius Severus. However, the cavalry bodyguards, like the Praetorians, fought on the side of Maxentius during the civil wars and as such Constantine abolished both after the Battle of the Milvian Bridge. The Lateran Palace on the other hand was already in existence and became property of Constantine when he married his second wife Fausta. He eventually gave the Palace to the Bishop of Rome.
Other members of Constantine’s family also have a presence in Rome’s buildings. His mother Helena has ties to the Santa Croce in Gerusalemme. The Basilica was built around the Palazzo Sessoriano, which she converted into a chapel in the AD 320s. The basilica was consecrated around AD 325 to house the relics of the Passion of Jesus Christ that Helena brought to Rome from the Holy Land. Santa Constanza can be found on the Via Nomentana, built as a Mausoleum forConstantine’s daughter Constantia (or his other daughter Helena), with Constantia’s sarcophagus being brought there later.