Llywelyn ap Gruffudd had been winning the long internal contest of thirteenth-century Wales for several years. Lords from Deheubarth, the southern Welsh kingdom - particularly Maredudd ap Rhys and Maredudd ap Owain - had sided with him. As a reward, Llywelyn had stripped lands from their nephew Rhys Fychan, who had backed the English crown, and given those lands to the loyalists. King Henry III viewed all this as a small Welsh quarrel. His son Edward, newly made Earl of Chester, saw it as a strategic problem and raised an army to do something about it. The plan was straightforward: land in south Wales, restore Rhys, intimidate Dinefwr Castle into surrender, and clip Llywelyn's wings before they grew larger. Stephen Bauzan, Lord of Breigan and Llansannor, would command in the field; Nicholas FitzMartin of Cemais would assist.

The English column moved up the Towy valley from Carmarthen, pillaging Welsh settlements as it went and aiming for Dinefwr. By Friday night the army was camped near Llandeilo Fawr. What they did not know was that two Welsh armies under the Marededdau - Maredudd ap Rhys and Maredudd ap Owain - had shadowed them through the woods on the surrounding hillsides. As the English settled into their fires the harassment began: arrows from the trees, javelins out of the dusk, hostile noises that gave the night no rest. Casualties mounted. Morale, already poor, began to crack. Stephen Bauzan sent Rhys Fychan - the army's guide and the man for whom this whole campaign had supposedly been mounted - to Dinefwr to negotiate the castle's surrender. Rhys either was seized inside the castle or simply went over to his cousins. Whichever it was, by morning the English were a column inside hostile country without a man who knew the paths.

On Saturday morning, lacking a guide, the English decided to retreat to Carmarthen. From the first hour of light until midday the Welsh shot at them out of the woods - bows, javelins, the kind of guerrilla pressure that does not destroy an army on its own but exhausts it utterly. At midday the two forces clashed at Coed Llathen, the "Wood of Llathen." The Welsh outflanked the English line and seized the supply train. Few were killed in this first engagement. But the English now had no provisions, no guide, and a long retreat ahead through country every yard of which the Welsh knew better than they did. The balance had shifted irrevocably.

The retreating column moved westward, and the country shifted with it. At Cymerau the ground turned wet and marshy, cut by ravines and choked with woodland. It was perfect Welsh country and impossible English country - knights on heavy horses cannot manoeuvre through bog, and a column strung out across uncertain ground cannot defend itself laterally. The Welsh attacked with their full army. The chronicles, written from Welsh memory, describe English knights pulled from their horses and trampled to death in the muck. Stephen Bauzan died there. The casualty count varies by source between 1,000 and 3,000 English dead. Llywelyn ap Gruffudd was said to have come down from Gwynedd in person, picking through the field afterwards for arms and armour. The survivors fled. Three thousand fathers and sons and brothers, by the higher count - if not Welsh, then certainly real, certainly someone's. The battlefield gave the campaign its name.

The victory at Cadfan let the Welsh take three English castles in quick succession - Laugharne, Llansteffan, and Narberth. Rhys Fychan hastily reconciled with Llywelyn and got his lands back. The Marededdau, having lost the new lands they had been given for backing Llywelyn, switched sides and paid homage to Henry III later in 1257. Henry himself, finally understanding that this was no small rebellion, organised another attack on Wales that same year. It failed - supply ships from Ireland never arrived, and the English were forced to retreat, harassed by victorious Welsh fighters the whole way back. Edward tried again in 1262 and was forced to pull out for political reasons at home. Llywelyn then mounted a devastating campaign against the Marcher Lords. By 1267, with Henry weakened by the Second Barons' War, the English crown opened negotiations - and the Treaty of Montgomery formally recognised Llywelyn as Prince of Wales. For one decade, until Edward I returned in earnest, Llywelyn ruled the most independent Wales had been since the Norman conquest.

The exact location of Coed Llathen and Cymerau is debated; the standard guesses place them in the woods and low ground between Llandeilo and Carmarthen, somewhere on the south side of the Tywi valley between modern Llandeilo and the village of Pontargothi. From the air today there is no mark of the battle. The valley still runs east to west between the same low hills. The trees still cover the slopes the Welsh armies came out of. Dinefwr Castle still stands on the ridge above Llandeilo, the strongpoint the whole campaign was supposed to break. It never broke.