Final answer:
Feudal monarchs in the High Middle Ages centralized their power by setting up government bureaucracies, replacing local noble and ecclesiastical officials with royal bureaucrats. This restructuring allowed monarchs to increase their direct control, eventually leading to more unified nation-states.
Step-by-step explanation:
During the High Middle Ages, feudal monarchs centralized power predominantly by setting up government bureaucracies. This process often involved the replacement of local vassals and church officials with royal bureaucrats who were loyal to the monarchy through direct salaries, rather than through feudal ties. French monarchs such as King Phillip II of the late twelfth century, for example, seized opportunities to add to their royal domains by confiscating lands from nobles. These new lands were then managed by royal appointees, curtailing the hereditary transmission of power among the nobility that characterized the feudal system and increasing the direct control of the monarch over their kingdom.
Additionally, the emergence of stronger central governments was further facilitated by the decline of noble privileges and the rise of the merchant class, which occurred partly due to events like the Black Death. The diminishing power of the nobility allowed monarchs to form the early foundations of modern centralized nation-states, thus changing the landscape from a fragmented realm of powerful lords to a more cohesive and centrally managed kingdom.