In many ways, the formation of modern Britain has been determined by its relationship with France. Perhaps the single most important date in this relationship was 1066, the date of of Norman Conquest.
The history leading up to 1066 begins with the Roman conquest of England under Julius Caesar, the subsequent collapse of the Roman Empire, and a series of incursion into and temporary conquests of England by various Scandinavian leaders, as well as intermarriages between Anglo-Saxon and European royalty. This meant that when Edward the Confessor died childless, England was claimed by three people, Harold Godwinson (brother-in-law of Edward), King Harald Hardrada of Norway, and William of Normandy (part of what is now France). William won the throne, and the Normans became the new aristocracy of England, with many older English aristocratic families fleeing to other parts of Britain.
Over subsequent centuries, this mingled heritage led to many territorial conflicts between France and England, exacerbated by England's becoming a Protestant country in the sixteenth century while France remained Roman Catholic.
Over the subsequent centuries, France and England were frequently at war, perhaps most dramatically when Napoleon planned to invade England but was eventually defeated and captured by the English. Although England and France, despite these hostilities were often trading partners, it was not until World War I that the French and English became allies, a geopolitical shift cemented in World War II. Since then, both countries have become members of the European Union, and have continued to cooperate militarily.