Having discussed the various types of troops, Machiavelli asserts that a prince "must have no other object or thought, nor acquire skill in anything, except war, its organization, and its discipline." The "art of war" must be the primary focus of a ruler. Military knowledge is "so useful" that "besides enabling hereditary princes to maintain their rule it frequently enables ordinary citizens to become rulers." Machiavelli writes, "The first way to lose your state is to neglect the art of war."
Machiavelli again emphasizes the overriding importance of military prowess, encouraging rulers to focus on "the art of war" before all other concerns. Military organization lays the foundation for the stability of the state and is a critical skill for both established and new rulers. There can be no security without military might.
To illustrate this principal, Machiavelli invokes the story of Francesco Sforza and his sons. With his knowledge of war, Sforza rose to become Duke of Milan. On the other hand, his sons, including Ludovico, "sank to being ordinary citizens after being dukes" because they neglected the art of war. If a prince is unarmed, he is "bound to meet misfortune" for obvious reasons, in addition to the fact that the "people despise you." A prince who does not understand warfare will not win the respect of his soldiers, and therefore he cannot place "any trust" in them.
As the story of Sforza and his sons illustrates, all princes must possess military prowess or risk losing their states. An unarmed prince is vulnerable not only to foreign invasions but also to the hatred of his people. The people will despise any prince who fails to protect them and thus ignores a crucial part of the social contract between rulers and subjects. A prince who does not protect his people cannot expect their loyalty in return.
Machiavelli urges a prince to study warfare "more vigorously in peace than in war." This study should be "both physical and mental," so that the prince's body and mind become accustomed to hardships. A prince should also learn "practical geography," a skill that can be easily applied to a variety of different provinces. In this way, the ruler will learn how to gain the tactical advantage in battle, regardless of the local geography.
Princes must engage in physical and mental exercises to enhance their military prowess. Through diligent study, princes can strengthen and build on their natural abilities. In peacetime princes must proactively study warfare in anticipation of future tests. Rulers must always prepare for the inevitability of war.
To complete his "intellectual training," a prince should read history, "studying the actions of eminent men" and learning from their successes and failures. The wise prince should model his behavior on "some historical figure who has been praised and honored." Machiavelli cites ancient leaders who have similarly modeled their actions on their eminent predecessors. Finally, a prince must "never take things easy in times of peace, but rather use . . . [it] assiduously, in order to be able to reap the profit in times of adversity."
Machiavelli advises princes to use peacetime to their military advantage. By preparing for war in times of peace, rulers will be ready for the challenges of wartime when it inevitably comes. Rulers should model themselves on the heroes of earlier generations, using their successes and failures to inform decision-making.