Denmark

The Dannebrog

This is reputedly, the oldest flag in use today by any independent nation, with its history being dated back to 1219. The legend states that during the battle of Lyndanisse, the battle was going badly for the Danish king. However, the Dannebrog flag was sent by God from the sky, and the king picked it up and inspired his men to victory. 

Before the use of the Dannebrog, the Danish would carry the Danish raven banner, which has been speculated as representative of Odin, the Viking King of the Gods.

Another story links the origins of the flag with the Pope sending it to the king for his crusades. However, the first recorded link goes back to a book of heraldic records dating back to the 1340's which links the white cross on red to the Danish king. 

There is also a State version of the flag, with the Splitflag, a swallow-tail version of the flag, being used by the State on land.

And the Orlogsflag is the State (Naval) flag to be used by Denmark at sea. It is differentiated from the Splitsflag by using a darker shade of red.

The Dannebrog is the inspiration for the design of the flags for the other Scandinavian nations of Finland, Sweden, Norway, and territories such as the Faroes.