“Maxentius organized his forcesstill twice the size of Constantine’sin long lines facing the battle plain, with their backs to the river. Constantine’s army arrived at the field bearing unfamiliar symbols on either its standards or its soldiers’ shields. According to Lactantius, Constantine was visited by a dream the night before the battle, wherein he was advised “to mark the heavenly sign of God on the shields of his soldiers...by means of a slanted letter X with the top of its head bent round, he marked Christ on their shields.” Eusebius describes another version, where, while marching at midday, “he saw with his own eyes in the heavens a trophy of the cross arising from the light of the sun, carrying the message, In Hoc Signo Vinces or “with this sign, you will conquer”; in Eusebius’s account, Constantine had a dream the following night, in which Christ appeared with the same heavenly sign, and told him to make a standard, the labarum, for his army in that form. Eusebius is vague about when and where these events took place, but it enters his narrative before the war against Maxentius begins. Eusebius describes the sign as Chi ;) traversed by Rho , a symbol representing the first two letters of the Greek spelling of the word Christos or Christ. The Eusebian description of the vision has been explained as a type of solar halo called a “sun dog”, a meteorological phenomenon which can produce similar effects.”
THAT WAS IN OCT 312 AD.
HE WON his military battle, he would have been useless against arians without his victory there.
St. Constantine was a Roman emperior and as such he lead numerous wars, including the one against Maxentius, a fellow Roman nobleman who was in rebellion. He converted to Christianity after he received the vision Eusebius describes. Maxentius was not an Arian and the war on him was not a war on the Arians. You knew that, didn’t you?