Auxiliary forces, or armies borrowed from an ally, are just as useless as mercenary forces because they fight with their own interests in mind. If they lose, the prince who utilizes them loses; if they win, the prince is under their favor. In victory, auxiliary armies are more dangerous than mercenary armies because they are united and capable of acting against their employers. Machiavelli provides numerous examples of princes who failed because they used auxiliary forces. As a reference, Machiavelli considers the example of Cesare Borgia. In his military campaigns, Borgia used all types of armies: auxiliary, mercenary, and his own. His accomplishments and reputation were guaranteed only when he relied on his own troops. Machiavelli reiterates this point through the example of Hiero of Syracuse and King David from the bible.
Hiero always used his own troops. And as David refused King Saul's armor, before going to battle against Goliath, Machiavelli warns, "In the end, the arms of another will fall from your hand, will weigh you down, or restrain you." Chapter 13, pg. 52
Machiavelli uses France as an example of a nation with an army of mixed composition-part Swiss mercenaries and part native. Because the French have come to rely upon the Swiss, they cannot do without them. If the French had its own army, they would be invincible. It is the folly of princes to consider the immediate benefits and forego the consequences. The decline of the Roman Empire is well known to have started with their reliance upon mercenary forces. It is no secret that great princes have always used their own armies.
