The Battle of Llandeilo Fawr took place during the conquest of Wales by Edward I, at Llandeilo between an English army led by Gilbert de Clare, 6th Earl of Hertford, and a south Welsh army.
Gilbert de Clare with an army of around 1600 infantry and 100 cavalrymen had captured Carreg Cennen Castle from the Welsh.
However, along the way Clare and his men were ambushed by Welsh troops and much of the army was destroyed.
English expansion into south Wales was halted for a few weeks and Edward's plans of an attack on multiple fronts were somewhat spoiled.
Following the defeat, Edward relieved Clare of his command and replaced him with Valence, who was now determined, following the death of his son.