This Triumphal Arch was dedicated in AD 315 to celebrate Constantine's victory three years before over his co-emperor, Maxentius.
Constantine claimed he owed his victory to a vision of Christ, but there is nothing Christian about the arch - in fact, most of the medallions, reliefs and statues were scavenged from earlier monuments. There are statues of Dacian prisoners taken from Trajan's Forum and reliefs of Marcus Aurelius, including one where he distributes bread to the poor.
Inside the arch are reliefs of Trajan's victory over the Dacians. These were probably by the artist who worked on Trajan's column.