There is typically a difference between a threat intended to make an adversary do something (or cease doing something) and a threat intended to keep him from starting something. The distinction is in the timing, in who has to make the first move, in whose initiative is put to the test. To deter by threat an enemy’s advance it may be enough to burn the bridges behind me as I face the enemy; to compel by threat an enemy’s retreat I have to be committed to move forward, and this requires setting fire to the grass behind me with the wind blowing toward the enemy. I can block your car in the road by placing my car in your way; my deterrent threat is passive, the decision to collide is up to you. If you, however, find me in your way and threaten to collide unless I move, you enjoy no such advantage; the decision to collide is still yours, and I enjoy deterrence. You have to arrange to have to collide unless I move, and that is a degree more complicated.

DEFINE: Draws the deterrence/compellence distinction: deterrence is passive (the adversary decides to collide), compellence requires the threatener to commit to moving, a more complicated arrangement.

Thomas C. Schelling, The Strategy of Conflict, loc. 438