According to the United Nations the flag of Denmark is the oldest flag still in use. Danneborg (Danish cloth) dates from 1219.
King Woldemar was leading his troops in battle against the Estonians but things were not going well for the Danes.
Reportedly the Danneborg floated down from the sky and the king grabbed it. At that time the tide turned against Estonia and the king and his army prevailed.
The flag, a red square, has a horizontal white stripe on the middle of the flag. The vertical stripe, also white, is placed left of center.
The flag was adopted 25 March 1757. The cross represents Christianity. The Knights of Malta have the same flag, they actually helped defeat the Estonians. White stands for honesty and peace, while red is symbolic of bravery, hardiness, valor and strength.
There was some controversy that the design of the Danneborg was linked to the Holy Roman Empire. However Denmark was never a province of the H.R.C.
Another theory of this flag is that the flag was sent to the king from the Pope. However another theory says that it wasn’t sent to the king but was a papal banner sent to the archbishop and that he then brought it along on the King’s crusade in the Baltic countries in an attempt to get the army to use a Christian symbol in battle.
This version doesn’t seem likely since the archbishop wouldn’t do this without the king’s permission.