Henry III (born 1207, r. 1216–1272), the eldest son of King John and Isabella of Angoulême, was critically important to the history of Magna Carta. Following King John’s death in 1216 Henry, who was just a young boy, inherited the throne in difficult circumstances. England was in the midst of civil war and on the verge of defeat to the French Prince Louis (later King Louis IX). The ‘minority government’ (ruling on young Henry III’s behalf) led by William Marshal, needed a way to tempt supporters of Prince Louis back to Henry’s side. A re-issue of Magna Carta was intended to do just this by conceding to the rebel barons the rights they had been fighting for in the first place.
In 1217, following the end of the civil war and Prince Louis’ peace settlement, Magna Carta was again issued in Henry’s name, together with an entirely new charter dealing with the royal forest. On 11 February 1225 King Henry III issued what became the final and definitive version of Magna Carta. It is clauses of the 1225 charter, not the charter of 1215, which are on the Statute Book of the United Kingdom today.
Further information about the life of King Henry III can be found via the Oxford Dictionary of National Biography.