In secular democracies, power is necessarily derived from the will of the governed. That power is then entrusted to a leader, who Machiavelli would understand to be a "prince". Inherently, his book, The Prince, has been close at hand for most politicians for centuries, as it provides general, historically proven advice for principalities and republics on how to govern and maintain relations with their most important resource and the very core of their power, which would be the people themselves.
Machiavelli's realization of the penultimate import of the people is probably most significant reason his book holds so much influence on realpolitik. He writes, "it is essential for a prince to possess the good will and affections his people, otherwise he will be utterly without support in time of adversity." (Chapter 9). Clearly, Machiavelli understands the source of power within a princely republic lay with the people, whom the prince must constantly court. No other political philosopher before him had ever given much significance to those being governed. The reason that Machiavelli felt that the subjects were vital to the prince maintaining his rule was because the implications of earning the hatred and ill will of the people are dire for the political future of both the state and the prince. Of the two sources of attack the prince must fear, one is a conspiracy from within inspired by the hatred of the people (Chapter 19). Additionally, the prince must be aware that actions of his intermediaries can reflect upon himself. That is, if his army is cruel and brutish towards the people, the people will turn their hatred upon the prince, who is seen to tacitly condone the actions of the army. All the advice that Machiavelli provides is given with the acknowledgment of the importance of the people. .