NovelGuide: The Prince: Novel Summary: Chapters 13-14

Chapter 13: This chapter continues the debate over the military.  Machiavelli separates mercenaries from auxiliaries, showing, again through historical example, that auxiliaries are much more dangerous because they are united.  While mercenaries only fight for money, auxiliaries often have ulterior motives that can backfire on the prince who hires them.  All the same, Machiavelli distrusts both kinds of foreign soldiers.  He counsels, "The wise prince, therefore, has always avoided these arms and turned to his own."
In addition to his many contemporary illustrations, Machiavelli even uses the Old Testament to support his rejection of foreign soldiers.  He cites David, who when told to put on Saul's heavy armor and weapons, instead preferred to use his own rags and slingshot.  Thus, the arms of others simply serve to "weigh down" a good warrior.  Likewise, the Roman Empire fell in part because Rome had contracted the services of barbaric mercenaries to the north.
Chapter 14: Next, Machiavelli underscores the importance of the study of military tactics for future princes.  Indeed the art of war is the key to success as a ruler, since unarmed princes are "despised." Furthermore, the prince, as commander-and-chief, must have a thorough grasp of military tactics if his soldiers are to respect and obey him.  He should also be physically fit and have a working knowledge of the local geography. He is not to be a lofty king who bathes himself in a castle, removed from his subjects, but someone who is connected with his land and his people.  Machiavelli continues, saying, "the prince that lacks this skill lacks the essential which it is desirable that a captain should possess, for it teaches him to surprise his enemy, to select quarters, to lead armies, to array the battle, to besiege towns to advantage."