Read the passage from chapter 14 of The Prince.

A prince ought to have no other aim or thought, nor select anything else for his study, than war and its rules and discipline; for this is the sole art that belongs to him who rules, and it is of such force that it not only upholds those who are born princes, but it often enables men to rise from a private station to that rank.

And, on the contrary, it is seen that when princes have thought more of ease than of arms they have lost their states. And the first cause of your losing it is to neglect this art; and what enables you to acquire a state is to be master of the art. Francesco Sforza, through being martial, from a private person became Duke of Milan; and the sons, through avoiding the hardships and troubles of arms, from dukes became private persons. For among other evils which being unarmed brings you, it causes you to be despised, and this is one of those ignominies against which a prince ought to guard himself, as is shown later on.

What type of evidence does Machiavelli use to support his claim that a prince should have no aim other than war and its rules and discipline?