A key part of A Distant Mirror, the Hundred Years’ War was not a continuous conflict lasting exactly 100 years. It was, as the book portrays, a complicated series of wars, truces, invasions, raids, diplomatic agreements, and renewed campaigns fought primarily between England and France from 1337 to 1453. The conflict originated in a combination of dynastic claims, territorial disputes, feudal obligations, economic interests, and competition for political influence. Its significance for A Distant Mirror is considerable, as warfare shaped the political institutions, economic conditions, aristocratic culture, and social instability of 14th-century Europe.

One major cause of the war was the English monarchy's possession of territories in France. English kings were simultaneously sovereign rulers of England and feudal lords possessing extensive French lands. This created an unstable relationship with the French monarchy. The English king was an independent monarch in England, but could theoretically be a vassal of the French king because of his French territories. Disputes over Gascony and other territories intensified this tension.

The dynastic issue became more serious after the death of the French king Charles IV in 1328. Edward III of England, grandson of Philip IV of France through his mother, Isabella, sought to claim the French crown. The French nobility instead supported Philip of Valois, who became Philip VI. Edward initially accepted Philip's position, but political and territorial conflicts eventually led to open warfare. In 1337, Philip confiscated Edward's French territories and Edward formally asserted his claim to the French throne.

The opening phase favored England. English naval forces won a major victory at Sluys in 1340, securing greater control of the English Channel. Edward III then invaded France and defeated Philip VI at Crécy in 1346 (whereupon Philip’s eldest son became King Jean II of France). The English captured Calais in 1347, establishing an important continental base. In 1356, Edward the Black Prince (eldest son of Edward III) defeated the French at Poitiers and captured King Jean II, an important moment in the narrative of A Distant Mirror. These victories demonstrated the effectiveness of English military organization and the longbow against traditional French cavalry tactics. The resulting Treaty of Brétigny in 1360 temporarily ended the first major phase of the conflict. England received extensive territorial concessions, while Edward III abandoned his formal claim to the French throne. The treaty, however, did not create lasting peace.

Under Charles V of France, the war changed direction. Rather than repeatedly seeking major pitched battles, the French adopted more cautious military strategies involving sieges, raids, and the gradual recovery of territory. By the time of Charles V's death in 1380, much of the English territorial expansion had been reversed. The later 14th century was characterized by intermittent warfare and truces, while both monarchies faced internal political difficulties.

These difficulties became particularly severe in France under Charles VI. The king experienced recurring episodes of mental illness, and political authority became increasingly divided among competing aristocratic factions. The most important were the Armagnacs and the Burgundians. Their rivalry eventually became intertwined with the English invasion, transforming the Hundred Years’ War into a complex civil and international conflict. The English monarchy also underwent political change. Richard II was overthrown in 1399 by Henry Bolingbroke, who became Henry IV. Henry V, his son, renewed the English campaign in France with remarkable success. In 1415, Henry V invaded Normandy and defeated a much larger French army at Agincourt. The battle became one of the most famous English victories of the war and reinforced the prestige of Henry and the English military system. 

Henry's subsequent campaigns placed France under enormous pressure. The internal conflict between Burgundians and Armagnacs weakened resistance, while the murder of John the Fearless, duke of Burgundy, in 1419 pushed the Burgundians toward an alliance with England. The result was the Treaty of Troyes in 1420. Charles VI recognized Henry V as his successor rather than his own son, the dauphin Charles. Henry V also married Charles VI's daughter Catherine of Valois. The agreement appeared to offer the possibility of an Anglo-French monarchy.

The settlement collapsed quickly. Henry V died in 1422, shortly before Charles VI, leaving the English claim to France in the hands of his infant son Henry VI. The dauphin Charles continued to claim the French throne. Political authority became divided between English-controlled territories and the regions loyal to Charles. The narrative of A Distant Mirror accelerates at this point as Tuchman’s main subject, Enguerrand de Coucy VII, dies.

The decisive transformation came with Joan of Arc. In 1429, Joan was instrumental in the French breaking of the English siege of Orléans, reversing the psychological momentum of the war. The dauphin Charles was subsequently crowned at Reims, strengthening his legitimacy as king of France. Joan was captured and executed in 1431, but the movement she helped initiate continued. The French gradually recovered territory, while the English position weakened.

The French recovery accelerated during the 1440s. French military organization became increasingly effective, and the development of artillery and more centralized royal forces reduced the strategic advantages that English armies had previously possessed. The war finally ended with the French victory at Castillon in 1453. England lost nearly all its possessions in France, retaining only Calais.

The consequences extended far beyond the battlefield. The war contributed to the growth of stronger centralized monarchies, altered the relationship between kings and aristocratic military elites, encouraged new forms of taxation and military organization, and contributed to changing ideas of political identity. In France, the eventual victory strengthened the monarchy and encouraged a more developed sense of French political unity. In England, the loss of continental territories redirected political attention toward the British Isles.

For Tuchman, the Hundred Years’ War is more than just a sequence of battles. It is a central mechanism through which the instability of 14th-century Europe becomes visible. Warfare disrupted agriculture, commerce, political authority, and ordinary life. It also reinforced (and eventually destroyed) the aristocratic culture of chivalry that Tuchman examines throughout A Distant Mirror. The war, therefore, provides the political and military framework within which her larger account of medieval society unfolds.