Constantine and the Cross (Costantino il grande in Italian) is a 1961 Italian production. The story takes place in the Dominate period of the Roman Empire at the beginning of Constantine's reign. In the early 4th century A.D., the Roman Empire was divided among four emperors or tetrachs with two emperors in the West and two in the East. In 303 A.D. Constantine became the emperor of Gaul.
Constantine's reign lasted from 306 to 337 A.D. but the movie goes as far as the Battle of the Milvian Bridge in 312 A.D. when Constantine went to war with emperor Maxentius one of the other tetrarchs. As a reminder, Constantine went to war with all the other tetrachs and took full control of the Roman Empire. He also went to war with the Franks and the Alamanni in 306-308 A.D..
Maxentius was fished out of the Tiber river, decapitated and his head was paraded through the streets of Rome (and later sent to Carthage so that there is no more resistance there). The movie actually contains quite many graphic images like the feeding of Christians to lions maybe to make a point about Rome's brutality before its conversion to Christianity.
The movie is about emperor Constantine's adoption of Christianity. Before the great battle, Constantine has a revelation and then tells his army that all Christians are allowed to worship their god. Note that a few months after the Battle of the Milvian Bridge, Constantine allowed the practice of Christianity throughout the empire with the Edict of Milan in 313 A.D..
When Constantine entered Rome, there was jubilation. He ended his procession at the Temple of Jupiter Capitolinus where usually a sacrifice is offered to the Roman god, a sign of his Christian beliefs. Constantine did not kill all of Maxentius supporters (which is what would be expected from a Roman emperor/general). However, he removed forever the Praetorian Guard and the Imperial Horse Guard and cancelled all legislation passed under Maxentus.
Overall, an entertaining movie if you do not mind graphic scenes. One of the rare ones that tells the story of Constantine, the emperor who legalized Christianity in Europe and who built a new Rome, in what used to be called Constantinople.