“There is a beautiful legend attached to the Dannebrog. Valdemar the Victorious (1170-1241) was on crusade to Estonia with his army. During the Battle of Lyndanisse on 15th June 1219, a red banner with a white cross came down from the heavens. The Danish army were victorious, and since that time this banner has been the Danish flag, which we call the Dannebrog.
The legend was first recounted by historians at the start of the 16th century, and since the start of the 17th century it has been firmly established in all subsequent histories of Denmark.”
Those words begins Hans Christian Bjergs review of the flag Dannebrog’s history, which through this 28-page booklet gives a good overview of the danish flag’s history.