In the medieval Welsh story of Dinas Emrys, the Red Dragon defeated the White Dragon underground.
The episode appears in the Historia Brittonum, a ninth-century historical compilation traditionally associated with Nennius. King Vortigern is trying to build a fortress at Dinas Emrys, but the walls repeatedly collapse. His advisers discover that a pool beneath the site contains two battling dragons.
The red dragon eventually overcomes the white dragon. The story interprets the battle as a prophecy: the red dragon represents the Britons, while the white dragon represents the invading Saxons. The episode helped establish the red dragon as a powerful Welsh national symbol.
This story is related to, but not identical with, the dragon on the modern flag of Wales. The flag’s red dragon draws on a long tradition of Welsh dragon imagery, while the Dinas Emrys account specifically centers on the underground battle and its political symbolism.