The year between 1276 and 1277 was not a good one for the prince of the Welsh. Llywelyn ap Gruffydd had badly misjudged the new king, and Edward reacted fiercely to the man who would not fulfill his obligations as a vassal. He immediately declared war on the Welsh, which was concluded by an English victory at Aberconwy in 1277. As punishment Edward took most of ap Gruffydd's lands, leaving him with only Lesser Gwynedd and the overlordship of five lords. Finally chastened, ap Gruffydd was at last allowed to marry the daughter of Simon de Montfort, to whom he had been betrothed since 1265. It was hoped the marriage would settle ap Gruffydd down. The marriage took place in 1278.
Disputes did arise regarding the use of English or Welsh law in the prince's lands, but the peace held until 1282 when ap Gruffydd's younger brother, Daffydd, attacked Harwarden. The prince, bound first by family loyalty, was forced to go to his brother's aid and was killed at Builth. He was buried at Cwm Hir, a Cistercian monastery he favored. His wife was already dead in childbirth, and his only legitimate heir was a sickly daughter, named for his sister, Gwynllian. She was put into a convent where she lived out her life. Daffydd ap Gruffydd was finally captured and executed in 1283.
When the Welsh told King Edward that they would only accept a prince who spoke no English, he cleverly presented them with his infant son, Edward, who had been recently born at Llywelyn ap Gruffydd's former stronghold of Caernavon. Thus ended Wales's last attempt at political independence. Until the year 1999.