The previous autumn, Wales had risen against the English king who had finished its conquest only twelve years before. Edward I's heavy taxation, his demand for Welsh troops to fight in Gascony, and the imposition of English law on Welsh communities all fed the rebellion. Madog ap Llywelyn, a distant cousin of the last native Prince of Wales, took the title of Prince of Wales himself and led a coordinated revolt across multiple regions in late 1294. Caernarfon Castle, only half-finished, was overrun and its constable killed. Edward had to abandon his Gascon campaign and march to Wales personally. Through the winter the king moved up the coast and around the north, and by early 1295 his commanders were closing on the surviving rebel forces. Madog withdrew east into mid-Wales, hoping to keep his army in being until summer.

Warwick was based at Welshpool. When his scouts brought word that Madog was camped in the upper Banwy valley to the west, he moved fast. The English column marched through the night of 4 March and reached Madog's position before dawn. The Welsh formed a square of spearmen on what the chroniclers called Maes Meidiog, the field of Meidiog. A cavalry charge by the English van came first and may have been repulsed, with around ten English men-at-arms killed. But Warwick had brought something the Welsh had not anticipated. He posted his archers and crossbowmen between his cavalry squadrons, alternating mounted and missile troops along the line, so that every time the English cavalry pulled back, a hail of arrows and bolts drove into the Welsh formation. The square that stopped horsemen could not stop projectiles. Men in the front ranks died standing where they had been told to stand.

When the Welsh formation broke, the survivors ran. The River Banwy was swollen with March snowmelt and they had to cross it to escape. Many drowned. The Annals of Worcester record that seven hundred Welshmen of noble rank were killed at Maes Moydog, drowned or fatally wounded, while Madog ap Llywelyn himself escaped only with difficulty. A second smaller engagement nearby, at a place the English called Thesseweit (the location now lost), cost Madog his supply train. The English put their own dead at about a hundred. The disparity tells the real story: a Welsh army that had marched and fought through a hard winter was finally caught in the open by a faster, better-equipped force using tactics the Welsh could not answer.

Madog was hunted across mid-Wales through the spring and early summer of 1295. He was captured in late July and ended his days in the Tower of London, where he is recorded as still alive in 1312. The revolt was over. Wales would not rise again for more than a century, until Owain Glyndwr in 1400. But Warwick's combination at Maes Moydog, cavalry and missile troops working together against massed infantry, was studied and remembered. English commanders refined the technique through the next generation, replacing crossbows with longbows and learning to dismount their knights to fight on foot behind a wall of archers. At Halidon Hill in 1333, and then at Crecy in 1346 and Agincourt in 1415, French knights would discover what those Welsh spearmen learned first on a flooded field above the Banwy: that the age when infantry could simply form a square and wait was ending. The medieval English military system that dominated the next two centuries had one of its first proper field tests on a hillside in Powys.