A Franco-British Union is a concept for a union between the two independent sovereign states of the United Kingdom and France. Such a union was proposed during certain crises of the 20th century; it has some historical precedents.
Historical unions
England and France
Ties between France and England have been intimate since the Norman Conquest, in which William the Conqueror, the French duke of Normandy, became king of England, while also owing feudal ties to the French crown.
The relationship was never stable, and it only endured as long as the French crown was weak. From 1066 to 1214, the king of England held extensive fiefs in northern France, adding to Normandy the counties of Maine, Anjou, and Touraine, and the Duchy of Brittany. After 1154, the King of England was also duke of Aquitaine (or Guienne), together with Poitou, Gascony, and other southern French fiefs dependent upon Aquitaine. Together with the northern territories, this meant that the King of England controlled more than half of France – the so-called Angevin Empire – though still nominally as the king of France's vassal. The centre of gravity of this composite realm was generally south of the English channel; four of the first seven kings after the Norman Conquest were French-born, and all were native speakers of French. For centuries thereafter the royalty and nobility of England were educated in French as well as English. In certain respects, England became an outlying province of France; English law took the strong impress of local French law, and there was an influx of French words into the English language.
This anomalous situation came to an end with the Battle of Bouvines in 1214, when King Philip II of France deposed King John of England from his northern French fiefs; in the chaos that followed, the heir to the throne of France, later Louis VIII, was offered the throne of England by rebellious English barons from 1216 to 1217 and travelled there to take it. He was proclaimed king of England in St. Paul's Cathedral, where many nobles, including King Alexander II of Scotland, paid him homage. He captured Winchester and soon controlled over half the kingdom, but after the death of King John his support dwindled and he was forced to make peace, renouncing his claim to the throne. England was ultimately able to retain a reduced Guienne as a French fief, which was retained and enlarged when war between the two kingdoms resumed in 1337.
From 1340 to 1360, and from 1369 on, the king of England assumed the title of "king of France"; but although England was generally successful in its war with France, no attempt was made to make the title a reality during that period of time.
The situation changed with King Henry V of England's invasion of France in 1415. By 1420, England controlled northern France (including the capital) for the first time in 200 years. King Charles VI of France was forced to disinherit his own son, the Dauphin Charles, in favour of Henry V. As Henry predeceased the French king by a few months, his son Henry VI was proclaimed king of England and of France from 1422 by the English and their allies but the Dauphin retained control over parts of central and southern France and claimed the crown for himself. From 1429 the Dauphin's party, including Joan of Arc, counterattacked and succeeded in crowning him as king.
Fighting between England and France continued for more than twenty years after, but by 1453 the English were expelled from all of France except Calais, which was lost in 1558. England also briefly held the town of Dunkirk in 1658–1662. The kings of England and their successor kings of Great Britain, purely as a habitual expression and with no associated political claim, continued to use the title "king of France" until 1801; the heads of the House of Stuart, out of power since 1688, used the title until their extinction in 1807.