1264

Events 
 Before May – Second Barons' War, a civil war in England, begins.
 May 12 to May 14 – The Battle of Lewes of the Second Barons' War is fought between Simon de Montfort, 6th Earl of Leicester and King Henry III of England in Sussex. By the end of the battle, de Montfort's forces capture both King Henry and his brother, future King Edward I, making de Montfort the "uncrowned king of England" for 15 months before Edward escapes captivity and recaptures the throne.
 June 18 – The Parliament of Ireland meets at Castledermot in County Kildare, the first definitively known meeting of this Irish legislature.
 In Spain, King James I of Aragon reconquers the cities of Orihuela in Alicante and Elx in Valencia from the Moors, ending over 500 years of Islamic rule.
 The Thuringian War of Succession ends.
 The state of Hesse gains its independence from Thuringia and becomes a free state of the Holy Roman Empire.
 In the Peerage of England, the title Baron de Ros, the oldest continuously held peerage title in England, is created by writ of summons.

 Births 
 May 26 – Prince Koreyasu, Japanese shogun (died 1326)
 Pope Clement V (died 1314)

 Deaths