Here is something I've sometimes wondered about. If I have a gun, and someone is attempting to steal something (either belonging to me or another), am I permitted to point the gun at her and command her, at gunpoint, to refrain from stealing. To fill out this case, suppose that I know the thief is not a physical danger to any person: I know the thief is unarmed and gentle. I am not interested in the legal question—that differs from jurisdiction to jurisdiction, and depends on what official role one plays. But can I morally use a threat of lethal force to prevent theft, assuming the law permits me to?
I assume that generally it is wrong to use lethal force to prevent mere theft (there are exceptions, such as cases where what is being stolen is something necessary to someone's survival). Now, pulling out the gun is offering a threat of force. Is it permissible to offer a threat that it would be immoral to carry out?
If one thinks deception is always wrong, the threatener is in the wrong. If she intends to carry out the threat, she is in the wrong since it is a threat that it is wrong to carry out. If she does not intend to carry out the threat, the threat is deceitful. However, while lying is always wrong, deceit is not. (There is nothing wrong with hanging a hat on a stick behind a bush to draw enemy fire.)
If deception is not always wrong, then one might think that with good reason it is permissible to make the threat. However, there is another consideration. It is wrong to blacken a person's reputation. But by making a threat it would be immoral to carry out, one is blackening a person's reputation—one is blackening one's own reputation, by making the other person think that one is the kind of immoral person that would use lethal force to protect property.
Still, the blackening of reputation is, perhaps, not intentional. (One may not be intending that the other person believe that one would carry out the threat, but only that the other person take herself to have reason to believe it.) So by Double Effect, the threat could, perhaps, be permitted in some cases.
If so, then it could in principle be permissible for a country to point nuclear weapons at enemy civilian centers as a deterrent, as long as the country could be sure that those weapons would not in fact be used against civilians. On the other hand, how could one be sure of that? Actually, even in the theft case, a similar danger exists: by making the threat, one creates a temptation in oneself to carry it out, and one should avoid temptation to do something immoral.