Inspired by true events
Since the 1200s the English crown restricted the Welsh from owning property in 'English' sections of wales, nor hold assemblies, nor hold military or civil positions in Wales.
In 1400, following a refusal to settle an unjust seizure of land solely because the offender was English, Owain ap Gruffyd revolted against English rule. In response, English King Henry IV restricted Welsh rights even more, prohibiting Welsh from owning land, nor could they carry armor. If a Welshman broke the law and fled to Wales, he could be immediately executed. No Englishman could be convicted of any crime against a Welshman in Wales.
These acts were understandably oppressive to the Welsh people, and further fueled support of Owain's cause.
In 1404, Owain ap Gruffyd was crowned Prince of Wales, and was recognized by the Welsh and the French as such, over Henry IV's eldest son, Henry of Monmouth.
In 1403, Henry of Monmouth, Prince of Wales in name, was seriously injured in a skirmish in Wales, and for a time was presumed dead by the Welsh. Although intelligence that he lived persisted, the young prince had mostly withdrawn from the public eye, and would not return to Wales for four years.
In 1407, Henry of Monmouth returns to Wales to claim his title as Prince of Wales from Owain ap Gruffyd. Owain is an experienced solider of over fifty, with support across his country. Henry is barely twenty, and his first major battle left him critically injured, besides which fact he has nearly no support from his father's impoverished regime.