The plague happened near the beginning of the conflict between England and France remembered as the Hundred Years’ War, which lasted from 1337 – 1453. That conflict was not really one war, but instead consisted of a series of battles and shorter wars between the crowns of England and France interrupted by (sometimes fairly long) periods of peace.
The war began because of simmering resentments and dynastic politics. The root of the problem was that the English kings were descendants of William the Conqueror, the Norman king who had sailed across the English Channel in 1066 and defeated the Anglo-Saxon king who then ruled England. From that point on, the royal and noble lines of England and France were intertwined, and as marriage between both nobles and royalty often took place across French - English lines, the inheritance of lands and titles in both countries was often a point of contention. The culture of nobility in both countries was so similar that the “English” nobles generally spoke French instead of English in day-to-day life.
This confusion very much extended to the kings themselves. The English royal line (the Plantagenets) often enjoyed pledges of fealty from numerous “French” nobles, and “English” kings often thought of themselves as being as much French as English - King Richard the Lion-Hearted, for instance, spent most of his career in France battling for control of more French territory. Likewise, a large region in southwestern France, Aquitaine, was formally the property of the English royal line, with the awkward caveat that, while a given English king might be sovereign in England, his lordship of Aquitaine technically made him the vassal of whoever the French king happened to be. Thus, hundreds of years after William’s conquest, the royal and noble lines of England and France were often hard to distinguish from one another.
The war began in the aftermath of the death of the French King Charles IV in 1328. The king of England, Edward III, was next in line for succession, but powerful members of the French nobility rejected his claim and instead pledged to give the crown to a French noble of the royal line named Philip VI. When Philip began passing judgments to do with the English-controlled territory of Aquitaine, Edward went to war, sparking the Hundred Years’ War itself.
The war itself consisted of a series of raids and invasions by English forces punctuated by the occasional large battle. English kings and knights kept the war going because it was a way to enrich themselves – they would arrive in France with a moderately-sized force of armed men to loot and pillage. English forces tended to be better organized than were their French counterparts, so even France’s much greater wealth and size did prevent major English victories. The most famous of those victories was the Battle of Agincourt in 1415, in which a smaller English force decimated the elite French cavalry through effective use of longbows, a weapon that could transform an English peasant into more than the equal of a mounted French knight. The aftermath of Agincourt saw most of the French nobility accept the English king, Henry V, as the king of France. Henry V promptly died, however, and the conflict exploded into a series of alliances and counter-alliances between rival factions of English and French nobles (one French territory, Burgundy, even declared its independence from France and became a staunch English ally for a time).
Decades into the war, the French received an unexpected boost in their fortunes thanks to the intervention of one of the future patron saints of France itself: Joan of Arc. Joan was an illiterate peasant girl who walked into the middle of the conflict in 1429, supporting the French Dauphin (heir) Charles VII. Joan reported that she had received a vision from God commanding her to help the French achieve victory against the invading English. French forces rallied around Joan, with Joan herself surviving several battles (her personal tactic consisted of praying for victory from horseback in the midst of battles - her survival was itself a major boost to French morale). Buoyed by the sense that God was on their side, French forces prevailed. Even though she was soon captured and handed over to the English for trial and execution as a witch by the Burgundians, Joan became a martyr to the French cause and, eventually, one of the most significant French nationalist symbols. By 1453, the French forces finally ended the English threat.
The war had a devastating effect on France. Between the fighting and the plague, its population declined by half. Many French regions suffered economically as luxury trades shut down and whole regions were devastated by the fighting. The French crown introduced new taxes, such as the Gabelle (a tax on salt) and the Taille (a household tax) that further burdened commoners. On the cultural front, the English monarchy and nobility severed their ties with France and high English culture began to self-consciously reshape itself as distinctly English rather than French, leading among other things to the use of the English language as the language of state and the law for the first time.