E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
In Latin, vincs in hoc; in English, By this conquer. It is said that Constantine, on his march to Rome, saw a luminous cross in the sky, in the shape and with the motto here given. In the night before the battle of Saxa Rubra a vision appeared to him in his sleep, commanding him to inscribe the cross and the motto on the shields of his soldiers. He obeyed the voice of the vision, and prevailed. The monogram is XP o (Christ). (See Gibbon: Decline and Fall, chap. xix. n.)
This may be called a standing miracle in legendary history; for, besides Andrews cross, and the Dannebrog or red cross of Denmark (q.v.), we have the cross which appeared to Don Alonzo before the battle of Ourique in 1139, when the Moors were totally routed with incredible slaughter. As Alonzo was drawing up his men, the figure of a cross appeared in the eastern sky, and Christ, suspended on the cross, promised the Christian king a complete victory. This legend is commemorated by the device assumed by Alonzo, in a field argent five escutcheons azure, in the form of a cross, each escutcheon being charged with five bezants, in memory of the five wounds of Christ. (See LABARUM.)