On this day, the enemy of Rome shall perish
Anticipating his rivals, Constantine crossed the Alps in 312 and sped his troops toward Rome. As Constantine made ready for battle on the evening of October 27, according to various reports he had a dream, or possibly a vision, where he saw an unusual sign emblazoned in the sky and either saw or heard the words Εν Τουτω Νικα (in this [sign], conquer). The sign incorporated the first two letters of the name of Christ in Greek: chi and rho. He made a new imperial standard, or labarum, and ordered his men to emblazon the same symbol on their shields. This is often rendered in Latin as In Hoc Signo Vinces, sometimes abbreviated as IHS.
Before committing himself to battle, Maxentius consulted the Sibylline books – the entry for October 28 read, “On this day, the enemy of Rome shall perish”. Maxentius declared this was a good omen and prepared to fight, certain that the gods were granting him victory.
Heavily outnumbered, the forces of Constantine charged the imperial army of Maxentius the next day, on October 28 at Saxa Rubra – the “Red Rocks” – some nine miles north of Rome. Maxentius was forced to fight with his back to the Tiber River, where the only possible retreat was over the narrow Milvian Bridge. Constantine's men routed Maxentius’ armies, forcing them to retreat across the Milvian Bridge. Many of the escaping troops were taken prisoner or killed, with Maxentius numbering among the dead.
When Constantine emerged victorious, the path of Western civilisation, and indeed, much of the world, as it had been known was changed forever.
O Lord, thy disciple Emperor Constantine,
who saw in the sky the Sign of Thy Cross
Accepted the call that came straight from Thee,
as it happened to Paul and not from any man
He built his capital and entrusted it to Thy care.
Preserve our country in everlasting peace,
through the intercession of the Theotokos,
for Thou art the lover of mankind.
Troparion of Ss Constantine and Helena (8th Tone)
Read more about the Great Sovereign and Equal-to-the-Apostles Constantine here.