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 (which at the time also encompassed County Durham), 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 only modifications have been regarding the Debatable Lands and Berwick-upon-Tweed). 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.

The treaty was one of a number of agreements made in the ongoing relationship between the two kings. The papal legate Otho of Tonengo was already in the Kingdom of England at Henry's request, to attend a synod in London in November 1237. Otho was informed in advance by Henry of the September meeting at York, which he attended. This meeting was recorded by the contemporary chronicler Matthew Paris, who disparaged both Alexander and Otho.

The Kingdom of England (Latin: Regnum Anglorum, "Kingdom of the English") was a sovereign state on the island of Great Britain from 12 July 927, when it emerged from various Anglo-Saxon kingdoms, until 1 May 1707, when it united with Scotland to form the Kingdom of Great Britain. The Kingdom of England was among the most powerful states in Europe during the medieval period.

On 12 July 927, the various Anglo-Saxon kingdoms were united by Æthelstan (r. 927–939) to form the Kingdom of England. In 1016, the kingdom became part of the North Sea Empire of Cnut the Great, a personal union between England, Denmark and Norway. The Norman conquest of England in 1066 led to the transfer of the English capital city and chief royal residence from the Anglo-Saxon one at Winchester to Westminster, and the City of London quickly established itself as England's largest and principal commercial centre.Histories of the kingdom of England from the Norman conquest of 1066 conventionally distinguish periods named after successive ruling dynasties: Norman 1066–1154, Plantagenet 1154–1485, Tudor 1485–1603 and Stuart 1603–1707 (interrupted by the Interregnum of 1649–1660).

Dynastically, all English monarchs after 1066 ultimately claim descent from the Normans; the distinction of the Plantagenets is merely conventional, beginning with Henry II (reigned 1154–1189) as from that time, the Angevin kings became "more English in nature"; the houses of Lancaster and York are both Plantagenet cadet branches, the Tudor dynasty claimed descent from Edward III via John Beaufort and James VI and I of the House of Stuart claimed descent from Henry VII via Margaret Tudor.

Following the conquest of England, the Normans gradually sought to extend their conquests both to the remainder of the British Isles and additional lands on the Continent, particularly in modern-day France. Over time, this would evolve into a long-standing policy of expansionism pursued intermittently with steadily increasing levels of aggression by successive, now-styled "English", dynasties. Beginning in the 12th century, the Normans began making serious incursions into Ireland. The completion of the conquest of Wales by Edward I in 1284 put Wales under the control of the English crown, although Edward's attempts to completely subjugate Ireland met with very limited success while the initial success of his conquest of Scotland was undone by English military defeat under his son, Edward II. Edward III (reigned 1327–1377) transformed the Kingdom of England into one of the most formidable military powers in Europe; his reign also saw vital developments in legislation and government—in particular the evolution of the English parliament. From the 1340s the kings of England also laid claim to the crown of France, but after the Hundred Years' War the English lost all their land on the continent, except for Calais. The subsequent outbreak of the Wars of the Roses in 1455 would ensure the English were never again in a position to seriously pursue their French claims.

After the turmoil of the Wars of the Roses, the Tudor dynasty ruled during the English Renaissance and again extended English monarchical power beyond England proper, in particular achieving the full union of England and the Principality of Wales in 1542. The Tudors also secured English control of Ireland, although it would continue to be ruled as a separate kingdom in personal union with England for centuries. Henry VIII triggered the English Reformation by breaking communion between the Church of England and the Roman Catholic Church, although the doctrinal aspects of the Reformation which established the English Church as being recognizably Protestant would not be pursued in earnest until the brief reign of his young son Edward VI. Following a return to Catholicism under the similarly brief reign of Henry's eldest daughter Mary I, Mary's half-sister Elizabeth I (reigned 1558–1603) re-established Protestantism under the terms of the Elizabethan Religious Settlement, meanwhile establishing England as a great power and laying the foundations of the British Empire by claiming possessions in the New World. While Henry also pursued an aggressive foreign policy north of the border in an attempt to subjugate Scotland, Elizabeth adopted a much more conciliatory position especially in light of developments such as Scotland's own Reformation and the eventual certainty that the Scottish monarch would succeed Elizabeth.

From the accession of James VI and I in 1603, the Stuart dynasty ruled England and Ireland in personal union with Scotland. Under the Stuarts, the kingdom plunged into civil war, which culminated in the execution of Charles I in 1649. The monarchy returned in 1660, but the Civil War had established the precedent that an English monarch cannot govern without the consent of Parliament. This concept became legally established as part of the Glorious Revolution of 1688. From this time the kingdom of England, as well as its successor states the Kingdom of Great Britain and the United Kingdom, have functioned in effect as a constitutional monarchy. On 1 May 1707, under the terms of the Acts of Union 1707, the kingdoms of England and Scotland united to form the aforementioned Kingdom of Great Britain.