From Discourses on the First Ten Books of Titus Livius, chapter 44, which is titled (in part) "It is not well to Threaten without Having the Power to Act":

From this we plainly see the folly and imprudence of demanding a thing, and saying beforehand that it is intended to be used for evil; and that one should never show one's intentions, but endeavor to obtain one's desires anyhow. For it is enough to ask a man to give up his arms, without telling him that you intend killing him with them; after you have the arms in hand, then you can do your will with them.