Convincing people to do that is a political task. It requires methods of leadership unknown or, as Machiavelli would prefer, forgotten in the Middle Ages. That is why Machiavelli insists on national armies, not mercenaries. He understands that soldiers in such armies need to be motivated. Modern politics are born from this necessity, and we moderns ignore it at our peril.
Enemies are always ready to march, or fly, or launch.
Machiavelli rejects the simplistic notion that war is a drastic departure from normal behavior. Having studied history, he knows that peace is rarer than war. Machiavelli tells us how to design and implement winning strategies. Like us, Machiavelli is saddened, frustrated, sometimes even enraged by the sight of mediocre leadership, that which is more corrupt than courageous, more self-indulgent than great in spirit. He knows from his study of history that men and women are often like that, but he also knows what greatness is and how it can be achieved by the best of us.