Final answer:
In the later Middle Ages, European knights transitioned from battling external invaders to engaging in internal conflicts and warfare among each other, due to the breakdown of the feudal system and pressures of land and honor. EUropean nobility and knights also continued to partake in Crusades or defend territories like Outremer, while the landscape of Europe shifted towards nation-state building and external trade warfare.
Step-by-step explanation:
Once Europe was no longer threatened by external enemies, European knights often attacked each other as a reflection of the feudal system's inherent violence. Without a centralized power or foreign invasions to unify them, feudal lords and their knights turned their attention inward, leading to raids on neighboring lands. The pursuit of warfare as a livelihood by lesser lords, and the emphasis on honor and retribution in feudal law, perpetuated a cycle of violence, initiating reprisals and civil wars for perceived slights. The medieval crusading orders and construction of defensive fortifications like Krak des Chevaliers also aimed at maintaining a European presence in Outremer. However, some European knights continued to venture abroad to participate in the Crusades in support of territories in the Holy Land, driven by familial obligations or religious fervor.
As the Middle Ages progressed, Christian kingdoms expanded internally and turned their efforts towards building up nation-states and engaging in dynastic conflicts. European powers also engaged in external warfare over trade and territory, further depleting their resources and contributing to the decline of the feudal system. The political landscape in Europe evolved from a series of confederations of warriors loyal to titular kings to a greater focus on nation-state building and consolidation of power.