“Maxentius organized his forces—still twice the size of Constantine’s—in long lines facing the battle plain, with their backs to the river.[153] Constantine’s army arrived at the field bearing unfamiliar symbols on either its standards or its soldiers’ shields.[154] 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.”[155] 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”;[156] 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.[157] Eusebius is vague about when and where these events took place,[158] but it enters his narrative before the war against Maxentius begins.[159] 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.[160] 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.
http://en.wikipedia.org/wiki/Constantine_I
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?