Simon de Montfort was born into the powerful Montfort family of France. His father, also Simon de Montfort, was the notorious leader of the Albigensian Crusade against the Cathars. The young Simon inherited his father's martial ambition and religious fervor, traits that would define his tumultuous career.

Simon's father was killed by a stone thrown from the walls during the Siege of Toulouse, ending the Albigensian Crusade. The elder Simon's death left the family's French domains contested and eventually lost, pushing the young Simon to seek his fortune elsewhere—ultimately in England.

Simon traveled to England to claim the earldom of Leicester, which his family held through his grandmother. King Henry III initially welcomed the young French nobleman and granted him possession of the Leicester estates, beginning a complex relationship that would dominate English politics.

Simon secretly married Eleanor of England, the king's sister and widow of William Marshal the Younger. The clandestine marriage—performed by the king's chaplain—caused scandal, as Eleanor had taken a vow of chastity. Henry III initially supported the union but later used it against Simon.

Facing baronial outrage over his secret marriage and debts, Simon briefly fled England. The English nobility resented a French upstart marrying into the royal family. This early exile revealed the tensions that would eventually explode into civil war.

Simon joined the Barons' Crusade to the Holy Land, fighting alongside other European nobles. His military prowess and piety enhanced his reputation. He returned to England with his status among the baronage significantly elevated.

Henry III appointed Simon as lieutenant of Gascony, the king's troubled French domain. Simon ruled with an iron fist, suppressing rebellious Gascon nobles through harsh measures. His effective but brutal governance created enemies and led to accusations of tyranny.

Gascon nobles brought charges against Simon before Henry III. Despite brilliant self-defense, the trial deepened the rift between Simon and the king. Simon felt betrayed by Henry's failure to support him, while Henry grew suspicious of Simon's ambitions and popularity.

Simon emerged as leader of the baronial reform movement that forced Henry III to accept the Provisions of Oxford. This revolutionary document established a council of fifteen barons to oversee the king and created regular parliaments. It was medieval England's first written constitution.

The reformers followed up with the Provisions of Westminster, extending protections to common people against baronial as well as royal abuses. Simon's support for these broader reforms distinguished him from self-interested barons and built his popular following.

With papal support, Henry III renounced the Provisions and began reasserting royal authority. Simon returned from France to lead the baronial resistance. The conflict polarized England between those supporting constitutional reform and those backing absolute royal power.

Open warfare erupted between Simon's baronial forces and the royalists. Simon proved a charismatic leader, attracting support from London, university scholars, friars, and common people who saw him as a champion of justice against tyrannical royal government.

Simon won a stunning victory at the Battle of Lewes, capturing King Henry III and Prince Edward. The Mise of Lewes established Simon as the de facto ruler of England. He governed in the king's name, implementing his vision of constitutional monarchy.

With the king his captive, Simon exercised royal power through a triumvirate. He attempted to balance reforming zeal with practical governance, but faced challenges from jealous barons, hostile marcher lords, and the captive Prince Edward's supporters plotting rebellion.

Simon summoned a revolutionary parliament that included not just barons and clergy but also knights from each shire and burgesses from selected towns. This first inclusion of commoners established a precedent that evolved into the House of Commons—Simon's greatest legacy.

Simon formed an alliance with Llywelyn ap Gruffudd, Prince of Wales, recognizing Welsh independence in exchange for military support. This controversial treaty with England's enemy alienated some English supporters and was used to portray Simon as a traitor.

Prince Edward escaped captivity with help from Gilbert de Clare, Earl of Gloucester, who had defected from Simon's cause. Edward quickly rallied royalist forces. The escape proved catastrophic for Simon, who found himself increasingly isolated and outmaneuvered.

Trapped by Prince Edward's forces at Evesham, Simon died fighting alongside his son Henry. His body was mutilated by his enemies, but he was venerated as a martyr by common people. His revolutionary parliament became the foundation of English constitutional government.