Pohnpei has discussed one of the key themes in the transition out of the Middle Ages into the Renaissance and Early Modern Era: the centralization of power under the monarchies. Indeed, political decentralization and fragmentation represents the definitive feature of the Feudal Power Structure. However, this raises an additional question: what caused European monarchies to start centralizing power in this fashion, or perhaps better phrased, what was it that enabled them to create these more centralized political structures?
Ultimately, the Medieval and Early Modern State was was closely tied to war. The requirements of Medieval warfare was at the heart of the Feudal System to begin with. Consider that, under the Medieval military structure, the Nobility served as the warrior class in society, and any given army was ultimately defined by the lords to call upon their various vassals, who themselves could often call upon their own forces. This was a fundamental rule which extended all the way up to the level of Kings. From this fundamental dependency came the so-called Feudal Contract, and what emerged was the decentralized Feudal System. In this system, monarchs ultimately relied on the cooperation of their vassals, who could themselves draw upon the military resources of their own vassals, and so on down the chain. By the start of the Renaissance, this old military structure was beginning to break down.
Consider the impact of gunpowder, which acted as a democratizing force on the battlefield and would come to undermine much of the Nobility's traditional role as a warrior elite (by the Early Modern Era, they would have taken on a new military identity, emerging as the new officer corp, responsible for overseeing and commanding the commoners who served as soldiers). In the Renaissance, warfare was becoming increasingly focused around mercenary companies, which themselves would later be replaced by the large Standing Armies of the Early Modern Era. This evolution eliminated the dependency which had previously chained Monarchs to the interests of the Nobility, which both enabled and incentivized monarchical centralization. Warfare and statecraft are very closely tied.