Constantine was the son of the western emperor Constantius Chlorus. When he died in 306, his father's troops proclaimed Constantine emperor. But in Rome, the favourite was Maxentius, son of Constantius' predecessor, Maximian.
Constantine overran Italy and faced Maxentius at the Milvian Bridge over the Tiber a few kilometres from Rome. Both knew it would be a decisive battle with Constantine's forces outnumbered.
But something strange happened. Eusebius, an early Christian historian, reports, "...about noon, when the day was already beginning to decline, he saw with his own eyes the trophy of a cross of light in the heavens, above the Sun, and bearing the inscription 'conquer by this'. "
Jens Ormo, a Swedish geologist, and colleagues working in Italy believe Constantine witnessed a meteoroid impact. The research team believes it has identified what remains of the impactor's crater. Radiocarbon dating puts the crater's formation at about the right time to have been witnessed by Constantine