Denmark’s national flag, the Dannebrog (the flag of the Danes), is the oldest flag in the world still in use by an independent nation, having been acknowledged in 1219.

The Flag of Denmark (Danish: Dannebrog Danish pronunciation: [ˈdanəˌbʁoːˀ] is red with a white Scandinavian cross that extends to the edges of the flag; the vertical part of the cross is shifted to the hoist side. A banner with a white-on-red cross is attested as having been used by the kings of Denmark since the 14th century. An origin legend with considerable impact on Danish national historiography connects the introduction of the flag to the Battle of Lyndanisse of 1219.

Source: https://en.wikipedia.org/wiki/Flag_of_Denmark