Compellence is a set of actions or positions that force an opponent to take some action desired by the initial actor. It is the opposite of deterrence, in which the actions are intended to prevent an opponent from taking some action. It is compellence when the classic lawman threatens a suspect with death if he does not surrrender; it is deterrence that inhibits the offender from initiating the action that would draw suspicion. Some consider it synonymous with coercion, and there are certainly such usages as coercive interrogation. In law, deterrence is similar to a restraining order, while coercion is more like a writ of mandamus, ordering a party to take some actions.
The term is often attributed to Thomas Schelling:
There is typically a difference between a threat intended to make an adversary do 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, and whose initiative is put to the test.
It is discussed in international relations theory, but, in general, is not common in widely available strategic literature. In a study at the United States Air Force, Lieutenant Colonel Michael Plehn observed the relative use of compellence and deterrence in U.S. policy documents:
|National Security Strategy of the United States of America (2002)||3||15|
|National Military Strategy||0||30|
|16 doctrinal documents of the Joint Chiefs of Staff and military services||58||250|
Plehn suggests that the disparity is an artifact of the Cold War, where nuclear deterrence was a necessity. He argues, however, that compellence is far more common in a multipolar world, in non-nuclear warfare, shows of force such as compelling the resignation of the Haitian junta in 1994, and in counterinsurgency.
The role of force
Bratton writes that the coercion literature can be divided into three "schools":
- "Coercive diplomacy" separate from the use of force, discussed by Alexander George and Janice Gross Stein: that which happens before "the first bomb is dropped". The actual use of force, as in deterrence, signifies failure, but the truly difficult part is exercising a signaling strategy understandable to all sides.
- coercion exercised almost entirely through the use of force (normally air power), in the works of Robert Pape, Daniel Byman, and Matthew Waxman; an example is the Second World War strategic bombing of Japan. A challenge here is that it is "hard to distinguish clearly between coercion and brute force given the scale and intensity of the conflicts studied."
- coercion exercised by both diplomacy and force, discussed by Schelling, Daniel Ellsberg, Wallace Thies, and Lawrence Freedman; an example is that of the "Lyndon Johnson administration to coerce the North Vietnamese government to cease its support of the Viet Cong insurgents in South Vietnam in the 1960s. In the view of a scholar who has traced this attempt, there is no sharp break between, first, coercive diplomatic efforts backed by very limited and covert use of force in 1963-64; second, limited demonstrative uses of force in reprisals after the Gulf of Tonkin incident; and third, the escalating air campaign of Operation ROLLING THUNDER."
Compellence by insurgents
The core of writing on compellence describes it as an interaction between nation-states. It was extended, in the Vietnam War to attempts to compel the Communist side, which had both conventional and guerrilla forces, with better results against the conventional than the guerrilla.
Plehn makes the point, however, thet "Coercion and insurgency are inextricably linked—in fact, insurgency is a form of compellence...Compellence is the use of influence to create a desirable outcome, or to prevent an undesirable outcome... On a similar vein, insurgency is a political tool that uses violence to affect or influence behavior—its essence is “protracted political violence.” Plehn defines "insurgency is the use—or threat—of violence by sub-national or unofficial organizations" to compel.
Compellence against insurgents
While the 2002 National Security Strategy abandoned compellence against violent non-state actors, it began to be reconsidered around 2006, but is still not a core doctrine. Thomas uses the term "coercion" to include both deterrence and compellence, and observes that coercion is discounted "in favor of killing militants today and draining the support swamp tomorrow. As a consequence, we forfeit potential options in the strategic space between development and destruction." 
- Patrick C. Bratton (Summer, 2005), "When is coercion successful? And why can't we agree on it?", Naval War College Review
- Thomas Schelling (1963), The Strategy of Conflict, Harvard University Press, p. 195
- Michael T. Plehn (May 2005), The Sharpest Sword: Compellence, Clausewitz and Counterinsurgency, Air Force Fellows program, Air University, United States Air Force, pp. 3-4
- United Nations Security Council Resolution 940
- Plehn, p. 55
- Steven Metz (2001), Counterinsurgency: Strategy and the Phoenix of American Capability, Strategic Studies Institute, U.S. Army War College
- Troy S. Thomas (2010), Beyond Pain: Coercing Violent Non-State Actors, National War College