The origin of Denmark's flag, known as the Dannebrog, is associated with a legendary event during the Battle of Lyndanisse in 1219. According to legend, as the Danish forces were struggling in the battle, a bright red flag with a white cross descended from the sky. The appearance of the flag inspired the Danish soldiers and led to a decisive victory against their enemies.

This event is not only significant in Danish history but also marks the flag as one of the oldest national flags still in use today. The Dannebrog has been an enduring symbol of Denmark and its heritage, representing the country's identity and unity.

For more detailed historical context, you can refer to sources like Visit Denmark or Denmark's National Museum.