The offer of compensation met a major royal grievance and it is suggested that only the work of Henry's son, lord Edward with Richard persuaded the King to reject the settlement. Indeed, according to the Song of Lewes, Edward demanded that the rebels hand themselves over with halters around their necks so that they may be ready for hanging and drawing. Richard ultimately rejected the form of peace as it would, in effect, disinherit the king of England and his heirs.
This was the fundamental problem: even the watered down Provisions as re-packaged the previous March during earlier negotiations continued to represent a major restriction on the crown's rights and powers. And following Henry's (so far) successful campaign throughout early 1264, what reason could the king have to now capitulate to them?
The following day, the 13th, with the offer of peace rejected with scorn by the royalists, Simon de Montfort's army closed on Lewes, to a location near Hamsey , with the possibility of drawing the royal army out to fight in the narrow gap between the Downs and the river. Thus "from the wood by Lewes" the letter of defiance was sent to the king. The rebels swore fidelity to Henry but declared their intention of "attacking according to our power our enemies who are also yours and those of all your kingdom".
Henry spurned these allegations of loyalty and declared the rebels to be his enemies. In an accompanying letter, Richard and Edward threw down their own challenge, vowing that they would do all they could to injure the persons and possessions of Montfort, Gilbert de Clare and their accomplices.
(These letters are dated 12th May, but it is almost inconceivable that they didn't follow de Montfort's letter of defiance on the 13th.)