Henry III was only nine years old at his accession in 1216, declaring himself of age in 1227. His grip on government and finance was weak and his dependence on foreign favourites generated resentment among the aristocracy, leading to the Marshal Rebellion of 1233-34.
His marriage in 1236 to Eleanor of Provence didn't help, and his promise to assist the Pope in Sicily provoked the barons to issue the Provisions of Oxford, which limited the king's power. Henry renounced these and the second Barons' War broke out in 1264, during which he was captured with his eldest son Edward at the battle of Lewes by Simon de Montfort, who ruled briefly until he died at Evesham in August 1265.
After that Henry little part in government and was succeeded by his son King Edward I in 1272.
Medieval Sourcebook: Simon de Montfort's Rebellion 1265 by Matthew of Westminster
England and its Rulers: 1066-1272, by MT Clanchy, at Amazon UK or Amazon USA
Kings and Queens of England
Chris's History Page