Henry III of England and Alexander II of Scotland signed the Treaty of York, establishing the Anglo-Scottish border, which mostly remains the same today.

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 John, King of England, and Isabella of Angoulême, Henry acceded to 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 the Great Charter of 1225, a later version of Magna Carta (1215), which limited royal power and protected the rights of the major barons. Henry's early reign was dominated first by William Marshal, and after his death in 1219 by the magnate Hubert de Burgh, Earl of Kent. 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 Catholic Church.

Alexander II was King of Alba (Scotland) from 1214 until his death. He concluded the Treaty of York (1237) which defined the boundary between England and Scotland, largely unchanged today.

The Treaty of York was an agreement between the kings Henry III of England and Alexander II of Scotland, signed at York on 25 September 1237, which affirmed that Northumberland, Cumberland, and Westmorland were subject to English sovereignty. This established the Anglo-Scottish border in a form that remains almost unchanged to modern times. The treaty detailed the future status of several feudal properties and addressed other issues between the two kings, and historically marked the end of the Kingdom of Scotland's attempts to extend its frontier southward.