Lactantius History of the Labarum and Milvian Bridge
You might like to read an account of the Labarum that was written much earlier that Eusibius' History. I have copied one chapter concerning it below. It comes from the Christian Lactantius ( http://www.newadvent.org/cathen/08736a.htm ) who was a tutor to Constantine’s son, and is written here in "Of the Manner in Which Persecutors Died" http://www.ccel.org/fathers2/ANF-07/anf07-15.htm#P3916_1567226. Chapters before this one give one a good history of the events that led up to the the Civil War and intrigue with Constantine’s own family.
And now a civil war broke out between Constantine and Maxentius. Although Maxentius kept himself within Rome, because the soothsayers had foretold that if he went out of it he should perish, yet he conducted the military operations by able generals. In forces he exceeded his adversary; for he had not only his father's army, which deserted from Severus, but also his own, which he had lately drawn together out of Mauritania and Italy. They fought, and the troops of Maxentius prevailed. At length Constantine, with steady courage and a mind prepared for every event, led his whole forces to the neighbourhood of Rome, and encamped them opposite to the Milvian bridge. The anniversary of the reign of Maxentius approached, that is, the sixth of the kalends of November, [27 Oct] and the fifth year of his reign was drawing to an end.
Constantine was directed in a dream to cause the heavenly sign to be delineated on the shields of his soldiers, and so to proceed to battle. He did as he had been commanded, and he marked on their shields the letter X, with a perpendicular line drawn through it and turned round thus at the top, being the cipher of Christ. Having this sign ( ), his troops stood to arms. The enemies advanced, but without their emperor, and they crossed the bridge. The armies met, and fought with the utmost exertions of valour, and firmly maintained their ground. In the meantime a sedition arose at Rome, and Maxentius was reviled as one who had abandoned all concern for the safety of the commonweal; and suddenly, while he exhibited the Circensian games on the anniversary of his reign, the people cried with one voice, "Constantine cannot be overcome!" Dismayed at this, Maxentius burst from the assembly, and having called some senators together, ordered the Sibylline books to be searched. In them it was found that:-
"On the same day the enemy of the Romans should perish."
Led by this response to the hopes of victory, he went to the field. The bridge in his rear was broken down. At sight of that the battle grew hotter. The hand of the Lord prevailed, and the forces of Maxentius were routed. He fled towards the broken bridge; but the multitude pressing on him, he was driven headlong into the Tiber.
This destructive war being ended, Constantine was acknowledged as emperor, with great rejoicings, by the senate and people of Rome. And now he came to know the perfidy of Daia; for he found the letters written to Maxentius, and saw the statues and portraits of the two associates which had been set up together. The senate, in reward of the valour of Constantine, decreed to him the title of Maximus (the Greatest), a title which Daia had always arrogated to himself. Daia, when he heard that Constantine was victorious and Rome freed, expressed as much sorrow as if he himself had been vanquished; but afterwards, when he heard of the decree of the senate, he grew outrageous, avowed enmity towards Constantine, and made his title of the Greatest a theme of abuse and raillery.
in hoc signo vinces
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