The flag of Denmark, also called the Dannebrog, is red with a Scandinavian cross on it.
According to a Danish legend, the flag just fell out of the sky during a battle, and they decided to make it their national flag. 
The Dannebrog is considered to be the oldest national flag still in use, and also the first national flag. However, the flag of Austria is older, as long as you discount the romantic legend about the origins of the Danish flag, as the first recorded use of the Danish flag was in the 1300s. 
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