Shortly after Prince Edward's birth, however, there was a falling out between the brothers-in-law.
Simon owed a great sum of money to Thomas II of Savoy, uncle of Queen Eleanor, and named King Henry as security for his repayment.
The King evidently had not approved this, and was enraged when he discovered that Montfort had used his name.
On 9 August 1239, Henry is reported to have confronted Montfort, called him an excommunicant and threatened to imprison him in the Tower of London. "
You seduced my sister", King Henry said, "and when I discovered this, I gave her to you, against my will, to avoid scandal."
Simon and Eleanor fled to France to escape Henry's wrath.

Having announced his intention to go on crusade two years before, Simon raised funds and travelled to the Holy Land during the Barons' Crusade, but does not seem to have faced combat there.
He was part of the crusading host which, under Richard of Cornwall, negotiated the release of Christian prisoners including Simon's older brother, Amaury.
In autumn 1241, he left Syria and joined King Henry's campaign against King Louis IX in Poitou in July 1242.
The campaign was a failure, and an exasperated Montfort declared that Henry should be locked up like King Charles the Simple.
Like his father, Simon was a soldier as well as a capable administrator.
His dispute with King Henry came about due to the latter's determination to ignore the swelling discontent within the country, caused by a combination of factors, including famine and a sense, among the English Barons, that King Henry was too quick to dispense favour to his Poitevin relatives and Savoyard in-laws.

In 1248, Montfort again took the cross with the idea of following Louis IX of France to Egypt.
But, at the repeated requests of King Henry, he gave up this project in order to act as the king's Lieutenant of the Duchy of Aquitaine (Gascony).
Bitter complaints were excited by the rigour with which Montfort suppressed the excesses of the Seigneurs and of contending factions in the great communes.
Henry yielded to the outcry and instituted a formal inquiry into Simon's administration.
Simon was formally acquitted on the charges of oppression, but his accounts were disputed by Henry, and Simon retired to France in 1252.
The nobles of France offered him the Regency of the kingdom, vacated by the death of Queen Blanche of Castile.
The earl preferred to make his peace with Henry III, which he did in 1253, in obedience to the exhortations of the dying Robert Grosseteste, Bishop of Lincoln.
He helped the King deal with disaffection in Gascony, but their reconciliation was a hollow one.
In the Parliament of 1254, Simon led the opposition in resisting a royal demand for a subsidy.
In 1256–57, when the discontent of all classes was coming to a head, Montfort nominally adhered to the royal cause.
He undertook, with Peter of Savoy, the Queen's uncle, the difficult task of extricating the King from the pledges which he had given to the Pope with reference to the Crown of Sicily; and Henry's writs of this date mention Montfort in friendly terms.
But at the "Mad Parliament" of Oxford (1258) Montfort appeared with the Earl of Gloucester, at the head of the opposition.
He was part of the Council of Fifteen who were to constitute the supreme board of control over the administration.
The King's success in dividing the barons and in fostering a reaction, however, rendered such projects hopeless.
In 1261, Henry revoked his assent to the Provisions of Oxford and Montfort, in despair, left the country.
Shortly after Prince Edward's birth, however, there was a falling out between the brothers-in-law. Simon owed a great sum of money to Thomas II of Savoy, uncle of Queen Eleanor, and named King Henry as security for his repayment. The King evidently had not approved this, and was enraged when he discovered that Montfort had used his name. On 9 August 1239, Henry is reported to have confronted Montfort, called him an excommunicant and threatened to imprison him in the Tower of London. "You seduced my sister", King Henry said, "and when I discovered this, I gave her to you, against my will, to avoid scandal." Simon and Eleanor fled to France to escape Henry's wrath.
Having announced his intention to go on crusade two years before, Simon raised funds and travelled to the Holy Land during the Barons' Crusade, but does not seem to have faced combat there. He was part of the crusading host which, under Richard of Cornwall, negotiated the release of Christian prisoners including Simon's older brother, Amaury. In autumn 1241, he left Syria and joined King Henry's campaign against King Louis IX in Poitou in July 1242. The campaign was a failure, and an exasperated Montfort declared that Henry should be locked up like King Charles the Simple. Like his father, Simon was a soldier as well as a capable administrator. His dispute with King Henry came about due to the latter's determination to ignore the swelling discontent within the country, caused by a combination of factors, including famine and a sense, among the English Barons, that King Henry was too quick to dispense favour to his Poitevin relatives and Savoyard in-laws.
In 1248, Montfort again took the cross with the idea of following Louis IX of France to Egypt. But, at the repeated requests of King Henry, he gave up this project in order to act as the king's Lieutenant of the Duchy of Aquitaine (Gascony). Bitter complaints were excited by the rigour with which Montfort suppressed the excesses of the Seigneurs and of contending factions in the great communes. Henry yielded to the outcry and instituted a formal inquiry into Simon's administration. Simon was formally acquitted on the charges of oppression, but his accounts were disputed by Henry, and Simon retired to France in 1252. The nobles of France offered him the Regency of the kingdom, vacated by the death of Queen Blanche of Castile. The earl preferred to make his peace with Henry III, which he did in 1253, in obedience to the exhortations of the dying Robert Grosseteste, Bishop of Lincoln. He helped the King deal with disaffection in Gascony, but their reconciliation was a hollow one. In the Parliament of 1254, Simon led the opposition in resisting a royal demand for a subsidy. In 1256–57, when the discontent of all classes was coming to a head, Montfort nominally adhered to the royal cause. He undertook, with Peter of Savoy, the Queen's uncle, the difficult task of extricating the King from the pledges which he had given to the Pope with reference to the Crown of Sicily; and Henry's writs of this date mention Montfort in friendly terms. But at the "Mad Parliament" of Oxford (1258) Montfort appeared with the Earl of Gloucester, at the head of the opposition. He was part of the Council of Fifteen who were to constitute the supreme board of control over the administration. The King's success in dividing the barons and in fostering a reaction, however, rendered such projects hopeless. In 1261, Henry revoked his assent to the Provisions of Oxford and Montfort, in despair, left the country.
Simon de Montfort, 6th Earl of Leicester