Henry became king under a regency; William Marshal, 1st earl of Pembroke, and later Pandulf acted as chief of government, while Peter des Roches was the king's guardian. At the time of Henry's accession, England was torn by civil war and partially occupied by the French prince Louis (later King Louis VIII). In 1217, however, the French were defeated and withdrew. Some of the English barons, Louis's former allies, continued to cause trouble; but Hubert de Burgh, chief justiciar and the greatest power in the government after 1221, gradually restored order.
In 1227, Henry was granted full powers of kingship, and in 1230, with typical willfulness and against the advice of the justiciar, he led an unsuccessful expedition to Gascony and Brittany. In 1232 the king dismissed Hubert de Burgh, and for the next two years the government was controlled by Peter des Roches and his nephew (or son), Peter des Rivaux. This administration, which consisted of trained civil servants (many of them Poitevin), was hated by the barons, and a baronial revolt (1233–34) forced Henry to dismiss it.
Henry then assumed direct control of the government, but despite frequent protests from the barons and from his brother, Richard, earl of Cornwall, the king continued to surround himself with French favorites, including relatives of Eleanor of Provence (whom he married in 1236) and his own Poitevin half-brothers. The latter involved him in a disastrous campaign (1242) to expel Louis IX of France from Poitou.
In 1238, Henry had weathered a storm of baronial protest caused by the secret marriage of his sister, Eleanor, to Simon de Montfort, earl of Leicester. The king subsequently (1248) sent Montfort to restore English authority in Gascony, but he totally alienated his former friend when he recalled him (1252) to answer charges of unjust administration.
In 1254, Henry accepted the papal offer of the kingdom of Sicily for his younger son, Edmund, earl of Lancaster (see Lancaster, house of), agreeing in return to finance the conquest of the kingdom from the Hohenstaufen dynasty. However, the English barons, disturbed by the king's subservience to the papacy (which had already resulted in large papal exactions and an influx of foreign clergy into England) and angry that they had not been consulted, refused the necessary funds. Threatened by the pope with excommunication, Henry was forced to come to terms with the baronial opposition, now led by Simon de Montfort. The king accepted its plan for conciliar government set forth in the Provisions of Oxford (1258), supplemented by the Provisions of Westminster (1259).
Divisions in the baronial party enabled Henry to repudiate (1261) the provisions, with papal sanction, and in 1263 war broke out (see Barons' War). An attempt to have Louis IX of France arbitrate the dispute led to the Mise of Amiens (1264), a declaration completely in the king's favor, and the war was renewed. Montfort won (1264) the battle of Lewes and summoned (1265) his famous representative Parliament. However, the heir to the throne, Prince Edward (later Edward I), led the royal troops to decisive victory at Evesham (1265), where Simon de Montfort was killed, and by 1267 the barons had capitulated. From 1267 on, Prince Edward actually ruled the realm, and Henry was king in name only.