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Constantine rides the white horse in gold armor and a crown. He holds one hand out over the fighting. Behind him the army's eagle standard sits in its wreath as it always did, and a cross is bolted on above it.

In October of 312 Constantine came down on Rome with a fast, lean army to take it from Maxentius, his brother-in-law and one of four men that year calling himself emperor. Maxentius held the city, the Praetorian Guard, and Aurelian's high walls, and had twice already won civil wars by simply staying inside them. This time he consulted the Sibylline books, and the keepers returned an oracle that on the coming day the enemy of Rome would perish. He read himself as Rome, marched his army out to the river, and put the Tiber at its back. The stone Milvian Bridge had been cut as a precaution, so his engineers laid a bridge of boats. The sign has two versions, both from men who knew Constantine. Lactantius, writing within a few years, says it was a dream on the eve of battle: mark the shields with the monogram of Christ, the chi and rho crossed. Eusebius wrote after the emperor's death. He says Constantine swore to him personally that he and the whole army saw a cross of light standing over the afternoon sun, with words attached: in this sign, conquer. Whichever it was, the shields that came down to the river that morning carried a mark no Roman legion had ever fought under. The battle itself was short. Constantine's cavalry broke the line, the line went backward into the river, and the bridge of boats did what bridges of boats do under a running army. Maxentius went into the Tiber in his armor and stayed. The river returned him the next day, and his head crossed the city on a lance. Then it was shipped to Africa, where his supporters needed convincing. Constantine entered Rome, disbanded the Praetorian Guard after three centuries of emperor-making, and within months had co-signed an edict making Christianity legal from one end of the empire to the other. Within his lifetime it was the religion of the court; within a century, of the state. The fresco of the battle stands in the Vatican, which is one way to state the outcome. Raphael won the commission for the hall and died in 1520 with the wall barely begun. His pupils Giulio Romano and Gianfrancesco Penni painted the cavalry, the drowning, and the gold emperor on the white horse over the following four years. They worked partly from the reliefs on Constantine's own arch. Overhead they added what no one at the river reported: angels in the air above the cavalry. In the palace of the popes, twelve centuries after the fact, the sky was not going to stay neutral.