The name of the red-white Danish flag in Danish (known as Dannebrog or Dannebrogen in most other Scandinavian languages as well). Dannebrog (= the cloth of the Danes) is by far the oldest Nordic ("Scandinavian") flag. Its cross-adorned design is the precursor of all other Nordic national flags -- the flags of Sweden, Norway, Finland, Iceland, the Faeroes and Åland -- which all depict a sidewise (lying-down) Christian cross. According to legend, the Dannebrog fell down from the sky in 1219, when the Danish army was in a particularly tight spot while fighting a battle with Estonians outside of what today is the Estonian capital Tallinn.
Happy times in Denmark
The Dannebrog flag itself and the more apocryphal legend of how it became the national symbol of Denmark is connected with the reign of the popular medieval Danish king Valdemar II Sejr (= victor). Valdemar was also given the additional epithet Lovgiver (= law-maker) -- it was during his reign that the old Danish province laws were written down and nationally enforced.
During the first part of Valdemar's reign (1202-1223) things looked truly bright in Denmark. The Öresund herring fisheries gave fabulous catches, rich new agricultural areas had been established, and many new towns and villages were founded. Even before being crowned in 1202, Valdemar had conquered important parts of Northern Germany, including the cities of Hamburg and Lübeck. The Danish expansion in Germany continued after the coronation, making new inroads for Danish trade, which prospered as never before.
There was a fly in all this lovely Danish ointment, however - the Estonians. Estonian seaborne marauders had made it a habit of raiding the Danish-held southern part of the Scandinavian peninsula (which is now southern Sweden). In 1202 the Estonians had landed in Blekinge and devastated the entire countryside. Valdemar decided to do something about it and got permission from Pope Honorius III to declare a crusade against the Estonians. After some initial fighting during the years preceding, the main Danish army, carried by some 500 ships, finally landed in Estonia in June 1219, close to where Tallinn is now situated.
The term "crusade" may have been just a pretext on King Valdemar's part, but the Danish clergy took its religious implications seriously. Hence the archbishop of Lund, Anders Sunesen, a learned man who had studied in Paris, Bologna and Oxford, together with an entourage of lesser clergymen, joined King Valdemar's expedition against the shamanistic Estonians.
In the gathering dusk of the evening of June 15, 1219, the Estonians attacked the Danish beachhead in force, in order to repel the Danish invasion. In the beginning things looked pretty bleak for the Danes. Undaunted, archbishop Anders Sunesen decided to call in reinforcements from above, raising his arms skyward. This had immediate effect -- the Danes were now successfully beating back the Estonian attacks. Unfortunately, after a while the Oxford-educated archbishop's arms, hitherto mainly used for pious paperwork, became tired. But whenever he lowered his arms, the Estonians again gained the upper hand in the battle.
Now it was time for the lesser clergy to enter the scene, by supporting the archbishops tired arms with their own. After a while of standing in this awkward position, the clericals suddenly saw how a red flag with a white cross came floating down from the sky, accompanied by the voice of God, commanding in His deep baritone: "Carry it high, and you will be victorious!". The Danes did as they were told, won the battle, and later conquered the northern part of Estonia.
Killjoys sometimes point out that this legend has suspicious-looking similarities with the Biblical legend in Exodus 17:8-16, where the Israelites win over some nasty tribe in a similar manner. In reality the Dannebrog is said -- by these prosaic individuals -- to have been a gift from Pope Honorius III, patterned after the flags of the Palestinian crusaders. And they also prosaically point to the fact that the legend itself is not from 1219, but arose a century or so later. This is most unfortunate, as the legend would make the Dannebrog the oldest still used national flag. But sadly, solid documentary proof of the earliest use of the Dannebrog dates to the 14th century.
The nice-looking Dannebrog flag, with its white cross on a red background, soon became the envy of Denmark's neighbors. When Sweden had established itself as a major Baltic power some centuries later, the Dannebrog design was copied by the Swedes, but with a yellow cross on a blue background. Later Norway followed, with a more complicated blue cross with white edges, set against a red background (it can also be seen as a Dannebrog with a blue cross added inside the white one). Finland designed its flag after the Danish model as well, with a blue cross against a white background. Iceland, the Faeroes and Åland complete the series of the Nordic cross-design national flags, although it might be argued that the Faeroes (a Danish province) and Åland (a Finnish province) are not proper nations, just self-governing provinces with an unusual amount of independence. Such arguments don't make their flags any less beautiful, when they wave valiantly in the wind against the light-blue Nordic sky, alongside all the other Dannebrog-derived flags of The North.
Ironically, the most secularized and least Christian countries in the Western world -- the Nordic countries -- are quite happy to display the Christian cross as their national symbol.