The end of the English kings came at the hand Simon de Montfort (son of a French crusader who became Earl of Leicester through his mother's bloodline), who himself married Henry III's sister Eleanor in secret. This was yet another point of strife in the kingdom as the barons of England protested the marriage, as Eleanor had been widowed of the Earl of Pembroke, and they demanded that their opinion on such an important marriage should have been asked even though Henry had given his permission. De Montfort and Henry themselves had a falling out when de Montfort used the king's name as security on a loan, and, after Henry discovered this, he told de Montfort, "You seduced my sister, and when I discovered this, I gave her to you, against my will, to avoid scandal." The feud caused Montfort and his wife to flee England in 1239, going on crusade and being offered the regency of France before returning in 1253 to make peace with Henry.
The peace would be a shallow one, however, as de Montfort began to lead the argument against Henry's demand for a subsidy of royalty from the barons. While de Montfort continued to support Henry on foreign affairs such as undoing pledges to the Pope, he determined that Henry's domestic policies were causing disapproval among all English, especially barons. In 1258, a parliament was called at Oxford, where de Montfort worked with the barons to ease the troubles between them and the king. There, he became more enfranchised with his fellow barons, but he did not approve wholeheartedly of the oligarchy created by the Provisions of Oxford, which gave the barons tremendous power in a Council of Fifteen to control domestic affairs. Henry was forced to take an oath on the Provisions, but, in 1261, he was granted a Papal Bull that nullified his vow. Civil war erupted three years later as the barons rallied under de Montfort to force the king to loosen his grip on the country, and the Battle of Lewes in 1264 gave a staunch victory to de Montfort when he captured both Henry and his son, Edward Longshanks.
With the king under guard and many of the barons his direct allies, de Montfort became the de facto ruler of England. He established a triumvirate with the Earl of Gloucester and the Bishop of Chichester, whom he controlled, and a new Parliament, which became unique in its inclusion of burgesses from economic boroughs as well as the knights of counties and in that de Montfort demanded all members be chosen by election, with the vote available to any man who owned land with the value of an annual rent of 40 shillings. The extension of power to the lower classes upset many of the barons, but de Montfort had hope in his state-building by unifying the peoples of England on a wide scale. Further barons distrusted de Montfort's alliance with Llywelyn, Prince of Wales, who took advantage of the English war to affirm the independence of Wales, weakening the Marcher Barons' holds there.
De Montfort felt his control of the nation slipping, so he decided to use power fully before it could vanish from him. Upon the opening of the Parliament, de Montfort pushed through a bill stripping Henry III of his title of king based on treason for canceling his oath from Oxford. A second bill established that England should have only a prince for its foreign affairs, which meant Edward Longshanks would never become more than a figurehead. Many of the barons balked, and several began to conspire against de Montfort, but he assured his legacy by promptly dispatching Edward from the kingdom to go on crusade and imprisoning Henry until his death. Edward, who had agreed with the Provisions of Oxford initially, determined he would be content until at least he was not surrounded by de Montfort's trusted (and armed) guards while in a foreign land.
Without a royal for his enemies to rally behind, de Montfort secured his power, primarily by his new enfranchisement of the growing middle class of England. When Longshanks arrived back from crusade in 1272 after the death of his father, he attempted to overthrow de Montfort's new permanent Parliament with barons who wished to gain back their power, but the grassroots support had grown firm and further aid flowed in from Llywelyn of Wales and Longshanks' brother-in-law, Alexander III, King of Scotland, who had also struggled against the power of an English king under Henry. Longshanks was again captured, and de Montfort stripped his title by act of Parliament as he had done with Henry, making Longshanks' quieter brother Edmund the new Prince of England. Edward Longshanks would live out his life under house arrest.
England settled into a sense of quiet prosperity and growing trade, sharing Britain with Wales and the Kingdom of Scotland, which underwent its own crisis after a string of deaths in 1286 and resulted in the leadership of the Guardians of Scotland. In 1306, Robert the Bruce became King of Scotland, and Scottish rule would eventually spread over the whole of the British Isles after the Black Death swept through the trade towns of England in the fourteenth century.
In reality, de Montfort did not attempt to destroy the kingship of England, merely spread the power among a new base. His plan weakened his ties with the barons, and, after Edward's escape in May of 1265, nearly all of de Montfort's support disappeared. Edward defeated and slew de Montfort at the Battle of Evesham and went on to become one of England's most powerful kings, confirming domination of Wales and invading Scotland in 1296.