The inscription on attic of the Arch of Constantine reads:
IMP CAES FL CONSTANTINO MAXIMO P F AUGUSTO S P Q R QUOD INSTINCTU DIVINITATUS MENTIS MAGNITUDINE CUM EXERCITU SUO TAM DE TYRANNO QUAM DE OMNI EIUS FACTIONE UNO TEMPORE IUSTIS REM PUBLICAM ULTUS EST ARMIS ARCUM TRIUMPHIS INSIGNEM DICAVIT
Translation:
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“To the emperor Flavius Constantine, the Great, pious and fortunate, the Senate and People of Rome, because by divine inspiration and his own great spirit with his army on both the tyrant and all his faction at once in rightful battle he avenged the State, dedicated this arch as a march of triumph.”
The “divine inspiration” was Constantine’s reported vision of the “sign” of Christ that would lead him to victory on the day of the battle. Eusebius references this in his Life of Constantine. The “tyrant” was Maxentius, his co-emperor of the Western Empire. With the defeat of Maxentius, Constantine became the sole emperor of a united empire.