This was civil war by another name. Between 1264 and 1268, Henry III struggled with the barons led by Simon de Montfort. The barons sought to circumscribe the king's powers in the ways described by the provisions of "Oxford of 1258 and the following year's provisions of "Westminster. In 1264 the king was captured by de Montfort at the Battle of Lewes, but in 1265 de Montfort was killed at the Battle of Evesham.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.