Machiavelli places a great deal of significance on an effective military, claiming in chapter 12 that the "main foundations of all States...are good laws and good arms." Yet he also recognizes that armies can be a disruptive force as well as a crucial bulwark of the power of the state.  For this reason, he advises against the use of mercenaries and especially auxilliaries, who he views as disloyal and self-interested:

Let him, therefore, who would deprive himself of every chance of success, have recourse to auxiliaries...bringing ruin with them ready-made.

The best possible military forces are what he calls "national forces," men who are the actual subjects of the state. They have much more of an interest in fighting, and are more likely to feel an allegiance to the prince. A well-trained, loyal army can be a great source of power for a wise ruler, but a mercenary force, or a disloyal military, can be the source of a state's destruction. Above all, a successful leader must become a good military leader, through both training and study.