As a rule, one must threaten that he will act, not that he may act, if the threat fails. To say that one may act is to say that one may not, and to say this is to confess that one has kept the power of decision—that one is not committed. To say only that one may carry out the threat, not that one certainly will, is to invite the opponent to guess whether one will prefer to punish himself and his opponent or to pass up the occasion. Furthermore, if one says that he may—not that he will—and the opponent fails to heed the threat, and the threatener chooses not to carry it out, he only confirms his opponent's belief that when he has a clear choice to act or to abstain he will choose to abstain

Thomas C. Schelling, The Strategy of Conflict, loc. 414