The reason Constantine was going to do battle the next day was to decide once and for all who should be the ruler of the Roman empire.
When his father, Constantius, had died six years earlier, he’d been proclaimed emperor. There was no doubt about that. The problem was that one of his generals, General Maxentius, had other ideas. He thought he should be emperor, and now that the matter was going to be settled by a battle close to a bridge that crossed the River Tiber near Rome.
The night before the battle, Constantine had a dream. Jesus appeared to him in the dream and said that he must make the cross his own symbol and have it placed on all the imperial flags and standards.
The next morning, October 28 in the year 312, the battle began. As in the army of General Maxentius began to advance across the bridge; it collapsed, flinging many of his soldiers into the river. Constantine won easily.
from The Loyola Treasury of Saints
Image credit: The Emblem of Christ Appearing to Constantine by Peter Paul Rubens, 1622. Public Domain via Wikimedia.