John's relationship with his nobles deteriorated further following the king's disastrous campaign to reconquer Normandy and Anjou from France in 1213, with the nobles eager to ally with Prince Llywelyn. Llywelyn's aid to England's nobles, in particular Llywelyn's seizure of Shrewsbury in May 1215, was one of the major factors which persuaded John to seal the "Magna Carta" in June 1215. Llywelyn wrestled significant concessions from the English Crown in the "Magna Carta". Land seized unjustly during the conflict would be returned to the other, and the primacy of Welsh law to apply in "Pura Wallia" (two-thirds of the surface area of Wales)- particularly with succession to land and title- was reaffirmed, and with "Marcher Law" to determine rights to land held in the March. The use of the term 'Marcher Law' in the "Magna Carta" was the first clear reference to the hybrid of Welsh and English law used in the march. The sealing of the "Magna Carta" did not end the conflict between John and England's nobles sparking First Barons' War, and Llywelyn continued to press his advantages in south and mid Wales by taking many English castles between 1215 and 1216, including the important Carmarthen and Cardigan castles among them.