The key to these threats is that, though one may or may not carry them out if the threatened party fails to comply, the final decision is not altogether under the threateneds control. The threat is not quite of the form "I may or may not, according as I choose," but, has an element of, "I may or may not, and even I can't be altogether sure." … Where does the uncertain element in the decision come from? It must come from somewhere outside of the threatener's control. Whether we call it "chance," accident, third-party influence, imperfection in the machinery of decision, or just processes that we do not entirely understand, it is an ingredient in the situation that neither we nor the party we threaten can entirely control.

Thomas C. Schelling, The Strategy of Conflict, loc. 416