Second Barons' War

Who was Simon de Montfort? | Henry III and the Second Barons' War > . 

The Provisions of Oxford were constitutional reforms developed in 1258 to resolve a dispute between the English barons and King Henry III. They asserted the right of the barons to representation in the king's government and, like the earlier Magna Carta, demonstrated the ability of the barons to press their concerns in opposition to the monarchy.

https://en.wikipedia.org/wiki/Provisions_of_Oxford .

https://www.britannica.com/event/Provisions-of-Oxford .

https://www.parliament.uk/about/living-heritage/evolutionofparliament/originsofparliament/birthofparliament/overview/simondemontfort/provisionsoxford/ . 

The Second Barons' War (1264–1267) was a civil war in England between the forces of a number of barons led by Simon de Montfort against the royalist forces of King Henry III, led initially by the king himself and later by his son, the future King Edward I. The war featured a series of massacres of Jews by Montfort's supporters including his sons Henry and Simon, in attacks aimed at seizing and destroying evidence of Baronial debts. After a rule of just over a year, Montfort was killed by forces loyal to the King in the Battle of Evesham.

https://en.wikipedia.org/wiki/Second_Barons%27_War .

https://en.wikipedia.org/wiki/Simon_de_Montfort,_6th_Earl_of_Leicester .

https://en.wikipedia.org/wiki/Henry_III_of_England .