Battle of the Milvian Bridge
This battle, fought in 312 outside of Rome, was between Constantine and Maxentius for control of the Roman Empire. The night before the battle, Constantine had a dream that if he painted Christ's symbol, Chi-Rho, on the shields of his soldiers he would win. He did so and was victorious so soon thereafter converted and became the first Christian Roman emperor. He also granted imperial tolerence of Christians, leading to more widespread Christianity.