After a decade, the population of the Perfeddwlad felt that their grievances were not being adequately addressed by Edward, and appealed to Llywelyn for assistance. Llywelyn was himself already aggrieved that when Llywelyn's younger brother, Dafydd, had reached adulthood, Henry had promised to re-divide Gwynedd so that Dafydd would also have lands to rule. So it was that in late 1256 Llywelyn invaded the Perfeddwlad, and captured it. Supply-line problems led to a series of defeats for the English, who subsequently had to abandon their campaign to deal with serious conflicts between the King and his Barons. In 1269, the King's weakened authority over the whole kingdom, and Llywelyn's gains in the meantime, lead to the Treaty of Montgomery, by which Henry acknowledged Llywelyn's gain of the Perfeddwlad, and his dominance over the rest of Wales - acknowledging Llywelyn as "Prince of Wales".