To persuade or dissuade a few from something, is very easy; for if words are not enough, you can use authority and force: but the difficulty is to take away a sinister idea from a multitude, whether it may be in agreement or contrary to your own opinion, where only words can be used, which, if you want to persuade everyone, must be heard by everyone.
Captains, therefore, must be excellent Orators, for without knowing how to talk to the entire Army, good things can only be done with difficulty. Which, in these times of ours, is completely done away with. Read the life (biography) of Alexander the Great, and see how many times it was necessary to harangue and speak publicly to the Army; otherwise he could never have them led them ((having become rich and full of plunder)) through the deserts of Arabia and into India with so much hardship and trouble; for infinite numbers of things arose by which an Army is ruined if a Captain does not know how or is not accustomed to talking to it; for this speaking takes away fear, incites courage, increases obstinacy, and sweeps away deceptions, promises rewards, points out dangers and the ways to avoid them, reprimands, begs, threatens, fills with hope, praises, slanders, and does all those things by which human passion are extinguished or enkindled. Whence that Prince or Republic planning to raise a new army, and to give this army reputation, ought to accustom the soldiers to listen to the talk of the Captain, and the Captain to know how to talk to them.
-Niccolo Machiavelli, The Seven Books on the Art of War. [An excerpt]