Barons' War, in English history, war of 1263–67 between King Henry III and his barons. In 1261, Henry III renounced the Provisions of Oxford (1258) and the Provisions of Westminster (1259), which had vested considerable power in a council of barons, and reasserted his right to appoint councilors. The barons led by Simon de Montfort, earl of Leicester, finally resorted to arms in 1263 and forced the king to reaffirm his adherence to the Provisions. In 1264 a decision in favor of the crown by Louis IX of France as arbitrator led to a renewal of war, but Montfort defeated Henry's forces in the battle of Lewes, and the king once again submitted to government by council. Early in 1265, Montfort summoned his famous representative Parliament to strengthen his position, which was threatened by the possibility of an invasion by Henry's adherents abroad. The invasion did not take place, but an uprising against Montfort of the Welsh
(Englishmen along the Welsh border) led to his defeat by the king's son (later Edward I) at Evesham. Montfort was killed in the battle, but some baronial resistance continued until 1267. The barons had failed to establish their own control over the crown, but they had helped prepare the way for the constitutional developments of the reign of Edward I.
See R. F. Treharne, The Baronial Plan of Reform (1932, repr. 1972); F. M. Powicke, King Henry III and the Lord Edward (1947).