Final answer:
The Hundred Years' War reduced the battlefield importance of knights, as the longbow and later firearms and gunpowder made the traditional cavalry charges ineffective. The rise of professional armies also decreased the military significance and social power of knights.
Step-by-step explanation:
The Hundred Years' War significantly altered the role of knights on the battlefield. The traditional dominance of cavalry, comprising knights from the noble class, became largely obsolete with the advent of new military technologies. Particularly, the use of the longbow, and later on firearms and gunpowder, changed warfare dynamics. These weapons could effectively target and eliminate knights before they could engage in close combat, greatly diminishing the knights' tactical advantage and status.
Additionally, the emergence of large, paid professional armies reduced the military reliance on feudal levies that the nobility could provide. This development diminished the social and political power of knights and other nobles. The ongoing conflict between England and France during the Hundred Years' War thus not only shaped national identities but also profoundly influenced military strategy and the structure of armies.