Henry III, also known as Henry of Winchester, was King of England, Lord of Ireland, and Duke of Aquitaine from 1216 until his death in 1272. The son of King John and Isabella of Angoulême, Henry assumed the throne when he was only nine in the middle of the First Barons' War. Cardinal Guala Bicchieri declared the war against the rebel barons to be a religious crusade and Henry's forces, led by William Marshal, defeated the rebels at the battles of Lincoln and Sandwich in 1217. Henry promised to abide by Great Charter of 1225, a later version of the 1215 Magna Carta, which limited royal power and protected the rights of the major barons. His early rule was dominated first by Hubert de Burgh and then Peter des Roches, who re-established royal authority after the war. In 1230, the King attempted to reconquer the provinces of France that had once belonged to his father, but the invasion was a debacle. A revolt led by William Marshal's son Richard broke out in 1232, ending in a peace settlement negotiated by the Church.

The monarchy of the United Kingdom, commonly referred to as the British monarchy, is the form of government used by the United Kingdom by which a hereditary monarch reigns as the head of state, with their powers regulated by the British Constitution. The term may also refer to the role of the royal family within the UK's broader political structure. The current monarch is King Charles III, who ascended the throne on 8 September 2022, upon the death of his mother, Queen Elizabeth II.