Simon de Montfort, 6th Earl of Leicester (c.1208 4 August 1265), later sometimes referred to as Simon V de Montfort to distinguish him from his namesake relatives, was a nobleman of French origin and a member of the English peerage, who led the baronial opposition to the rule of King Henry III of England, culminating in the Second Barons' War. Following his initial victories over royal forces, he became de facto ruler of the country, and played a major role in the constitutional development of England.

During his rule, Montfort called two famous parliaments. The first stripped the king of unlimited authority, while the second included ordinary citizens from the towns. For this reason, Montfort is regarded today as one of the progenitors of modern parliamentary democracy. As Earl of Leicester he expelled Jews from that city; as he became ruler of England he also cancelled debts owed to Jews through violent seizures of records. Montfort's party massacred the Jews of London, Worcester and Derby, killing scores of Jews from Winchester to Lincoln. After a rule of just over a year, Montfort was killed by forces loyal to the king in the Battle of Evesham.

The Battle of Lewes was one of two main battles of the conflict known as the Second Barons' War. It took place at Lewes in Sussex, on 14 May 1264. It marked the high point of the career of Simon de Montfort, 6th Earl of Leicester, and made him the "uncrowned King of England". Henry III left the safety of Lewes Castle and St. Pancras Priory to engage the barons in battle and was initially successful, his son Prince Edward routing part of the baronial army with a cavalry charge. However, Edward pursued his quarry off the battlefield and left Henry's men exposed. Henry was forced to launch an infantry attack up Offham Hill where he was defeated by the barons' men defending the hilltop. The royalists fled back to the castle and priory and the King was forced to sign the Mise of Lewes, ceding many of his powers to Montfort.