|The remains of Lewes Castle|
Henry lived a lavish lifestyle that demanded lots of money, which he demanded from increased taxation. He also got involved in issues that the English barons felt were not necessary to England's interests. For instance, Henry "bought" the title of "King of Sicily" from Pope Innocent IV by funding a war with the rulers of Sicily at that time, the Hohenstaufens. Henry wanted the title for his second son, Edmund. When funding for the war ran out, and victory was no longer in sight, Innocent reneged on the deal and gave the title to Charles of Anjou.
The barons, led by Simon de Montfort, tried to control Henry with, among other things, the 1258 Provisions of Oxford, reducing his powers. Henry got a dispensation from the pope in 1261 that released him (in his eyes) from the Provisions, and the barons and the forces loyal to Henry started arming for civil war.
A turning point came on 14 May 1264, at the Battle of Lewes. Henry was ensconced in Lewes Castle in Sussex, but left it to attack de Montfort's forces. Part of Henry's army was under his command, and part under that of Prince Edward (later King Edward I). Edward's cavalry drove off the attackers, but Edward pursued them, leaving his father outnumbered by the remaining de Montfort men. Captured, Henry was forced to sign an agreement called the Mise* of Lewes, giving control of government over to Simon de Montfort.
Constitutional historians do not recognize de Montfort as a king, even though he was controlling the government for about a year. His support faded, and Prince Edward raised an army a year later and defeated de Montfort at the Battle of Evesham, returning power to Henry.
*"Mise" is a rarely used word from French meaning "settlement."