Having laid out the characteristics of different principalities and having considered some of the varying reasons for their success or failure, Machiavelli turns to a discussion of the ways in which states can prepare themselves for attack or defense. Machiavelli again emphasizes that a prince must build on "sound foundations" or he will "come to grief." The main foundations of all states "are good laws and good arms." Because it is impossible to have good laws without good arms (military power), and because good laws inevitably follow from good arms, Machiavelli devotes his attention to military organization and does not discuss laws.
Although Machiavelli acknowledges the shared importance of laws and arms, he places a clear emphasis on the maintenance of good arms. While both good laws and good arms form the foundation of a stable state, Machiavelli advises princes to focus first and foremost on military organization, which will pave the way for legal and civic institutions. Rulers must prioritize military matters; therefore, Machiavelli focuses his discussion on good arms.
Among the different types of arms that a prince may use for his defense, there are his own troops, mercenary troops, auxiliary troops, and composite troops. Machiavelli explains that mercenary and auxiliary troops "are useless" and he encourages rulers to avoid employing them. Machiavelli states that mercenary troops are dangerous because "there is no loyalty or inducement to keep them on the field apart from the little they are paid . . . [which] is not enough to make them want to die for you." Machiavelli believes that mercenaries are more likely to desert and he blames "the present ruin of Italy" on its rulers' reliance on mercenary forces. When Charles VIII of France invaded and conquered Italy, he showed the Italians that their mercenaries are worthless and the Italians suffered the consequences for their misguided dependence on these forces.
With these statements, Machiavelli declares that mercenary troops cannot be included under the designation of "good arms." Lacking the loyalty of a citizen-raised army, mercenary troops are unreliable and ruinous. Reliant on a ruler only for their wages, mercenary soldiers lack the deep devotion and dependence that makes citizens armies into formidable opponents. Italy's reliance on these "bad arms" has made it impossible for rulers to lay strong foundations for their states.
Mercenary commanders "are either skilled in warfare or they are not," but in both instances they lead to ruin. If they are skilled, then they are more anxious to advance their own agenda than to serve their employer. If they are not skilled, then they will lose battles and ruin the prince "in the normal way." Machiavelli cites the Romans, Spartans, and Swiss as examples of states that maintained their sovereignty by raising their own armies. By contrast, the Carthaginians were nearly ruined by their reliance on mercenaries. Machiavelli offers Francesco Sforza as a modern example of the danger of mercenary troops, recounting how Sforza, a mercenary of Milan, eventually subjugated his employers and installed himself as the duke of Milan. Machiavelli does concede that the Florentines and Venetians have achieved some conquests with mercenary troops, although he attributes these successes to luck.
Rulers who hire mercenary troops will find themselves in a lose-lose situation. If a mercenary commander possesses prowess, then he will attempt to overthrow the current prince and claim power for himself. However, if the commander isn't skilled, then he will simply lose battles at the prince's expense. Machiavelli cites the highly skilled Sforza, himself a mercenary, to illustrate the danger of mercenaries. Through a combination of prowess and fortune, Sforza bridged the gap between the masses and the elite, using skill and opportunity to become the duke of Milan. Wise rulers must avoid the temptation of mercenary armies.
Discussing the Venetians' use of mercenaries, Machiavelli tells the story of how Venice, in "one day's engagement," lost the territory that it had taken them "eight hundred years' exertion to conquer." With this example, Machiavelli declares, "Mercenary armies bring only slow, belated, and feeble conquests, but sudden, startling defeat." Machiavelli then turns to the history of other Italian states that have relied on mercenaries, arguing that the priests-turned-popes and the citizens-turned-princes who have come to dominate Italy rely on mercenaries due to their lack of experience in military matters. As a result of Italian's reliance on foreign troops, the French, Spanish, and Swiss have overrun Italy. The Italians' mercenary troops have fought weakly, attempting to avoid "both exertion and danger" and therefore failing in the war. Machiavelli concludes that mercenaries "have led Italy into slavery."
Machiavelli blames Italy's current ruin on the inexperience of its novice rulers, who lack the military prowess necessary to lay strong foundations for their states. These rulers' lack of prowess contributes to their reliance on foreign arms, which undermines the stability of their principalities. Machiavelli strongly advises rulers to avoid the use of mercenaries, since they prioritize their own wellbeing over the interests of their princely employer. According to Machiavelli, ruin is the inevitable result of a prince's continued dependence on mercenaries.