On 16 September 1400, supporters of Owain Glyndŵr proclaimed him Prince of Wales. The declaration marked the beginning of a rebellion against English rule that continued for more than a decade.

Glyndŵr was a Welsh landowner from a family connected to the old Welsh princely houses. The immediate causes of his revolt included a long-running land dispute with his English neighbour, Reginald Grey, Lord Grey of Ruthin. Glyndŵr believed his attempts to obtain justice through English institutions had failed.

The dispute came amid wider Welsh resentment of English rule. After the proclamation, Glyndŵr and his supporters attacked English-held towns in north-east Wales. The uprising spread, and Glyndŵr gained support across much of the country.

By 1404, his position was strong enough for him to hold a parliament at Machynlleth. His plans went beyond removing English officials. He proposed Welsh universities, sought an independent Welsh church and developed diplomatic relations with France.

The rebellion eventually lost ground as English forces recovered territory and Glyndŵr’s alliances weakened. Harlech Castle, one of his principal strongholds, fell in 1409. Glyndŵr was never captured, and his final years remain uncertain.

The proclamation of 16 September 1400 is still commemorated in Wales, where many observe the date as Owain Glyndŵr Day.