Llewelyn the Last attacks English forces in South Wales.
22 August - Truce agreed between England and Wales.
1261
Early - Following disputes, northern academics from the University of Cambridge attempt to set up a university at Northampton, suppressed by the Crown in 1265.
23 January - King Louis IX of France issues the Mise of Amiens, a settlement between King Henry III of England and barons led by Simon de Montfort heavily favouring the former, which leads to the Second Barons' War.
February - Second Barons' War begins as Henry returns to fight Simon de Montfort's rebels.
12 May to 14 May - Second Barons' War: The Battle of Lewes is fought between Simon de Montfort and King Henry III in Sussex. By the end of the battle, Montfort's forces capture both King Henry and his son, Prince Edward, and Henry is forced to sign the Mise of Lewes making Montfort the "uncrowned king of England".
June - Simon de Montfort summons a parliament in London, the first to include Knights of the Shire.
12 August - Peace of Canterbury: Papal legate and King Louis IX of France condemn the rebels, who are later excommunicated.
In the Peerage of England, the title Baron de Ros, the oldest continuously held peerage title in England, is created by writ of summons.
1265
20 January - Montfort's Parliament, the first to include burgesses, and to insist that members be elected, assembles at Westminster.
28 May - Second Barons' War: Prince Edward escapes from captivity and rejoins royalist forces.
22 June - Simon de Montfort signs a treaty with Llewelyn the Last, recognising his rule over Wales.
4 August - Second Barons' War: The Battle of Evesham is fought in Worcestershire, with the army of Edward defeating the forces of rebellious barons led by Simon de Montfort and killing Montfort and many of his allies.
16 September - Second Barons' War: Henry disinherits all rebels against his rule.
1266
June - Second Barons' War: Henry III besieges the rebels in Kenilworth Castle.
31 October - Second Barons' War: The war winds down as supporters of the slain rebel leader Simon de Montfort make an offer of peace to the king in the Dictum of Kenilworth.
14 December - Kenilworth surrenders.
1267
9 April - Second Barons' War: Gilbert de Clare, 6th Earl of Hertford occupies London.
June - Second Barons' War: Prince Edward captures the Isle of Ely, and the remaining rebels surrender.
Summer - The Second Barons' War: Rebels and King Henry III agree to peace terms as laid out in the Dictum of Kenilworth.
19 November - The Statute of Marlborough is passed, confirming the Magna Carta and the Provisions of Westminster. It is the oldest English law still (partially) in force.
Roger Bacon completes his work Opus Majus and sends it to Pope Clement IV, who had requested it be written; the work contains wide-ranging discussion of mathematics, optics, alchemy, astronomy, astrology and other topics, and includes what some believe to be the first description of a magnifying glass. Bacon also completes Opus Minus, a summary of Opus Majus, later in the same year.
