What is a Triumphal Arch?

A triumphal arch is a monument used to celebrate a military victory. The first arches were simple arches with one passage and sometimes adorned by reliefs and attached columns. The older arches were more elaborate, both in decoration and form.

The Arch of Constantine:

The arch of Constantine is a triumphal arch. It was constructed in 315 CE to commemorate the victory of Constantine I, in the battle of the Milvian Bridge against Maxentius in 312 CE. The arch is located in Italy, Rome, in the valley of the coliseums, between the Palatine Hill and the Coliseum, along the road taken by the triumphal processions.

The Battle of Milvian Bridge:

Was the final battle between Constantine and Maxentius. Constantine and Maxentius both claimed the title of Augustus. Agustus was the legal title to ruler of Rome at the time. The conflict was finally resolved at the battle of Milvian Bridge.
The Milvian Bridge was built by Marcus Aemilius Scaurus in 109 BCE over the Tiber near Rome as a part of the Flaminian Way.