Chapter 1

Legal Bases For The Use Of Force

I.  Introduction

In both customary and treaty law, there are a variety of internationally-recognized legal bases for the use of force in relations between States.  Generally speaking, modern jus ad bellum (the law governing a State’s use of force) is reflected in the United Nations (UN) Charter.[1]  The UN Charter provides two bases for a State’s choice to resort to the use of force:  Chapter VII enforcement actions authorized by the UN Security Council, and self-defense pursuant to Article 51 (which governs acts of both individual and collective self-defense).

A.   Policy and Legal Considerations

1.    Before committing U.S. military force abroad, decision makers must make a number of fundamental policy determinations.  The legal aspects of such a decision, both international and domestic, are of primary concern in this determination.  Any decision to employ force must rest upon the existence of a viable legal basis in international law, as well as in domestic law (including application of the 1973 War Powers Resolution (WPR), Public Law 93-148, 50 U.S.C. §§ 1541-1548).

2.    Though use of force decisions will normally be resolved at the national political level, Judge Advocates (JAs) must understand the basic concepts involved in a determination to use force abroad.  Using the mission statement provided by higher authority, JAs must become familiar with the legal justification for the mission and, in coordination with higher headquarters, be prepared to brief all local commanders on that legal justification.  This effort will enable commanders to better plan their missions, structure public statements, and conform the conduct of military operations to U.S. national policy.  It will also assist commanders in drafting and understanding mission specific Rules of Engagement (ROE), which must be tailored to calibrate the authority to use force consistent with national security and policy objectives.

3.    JAs must also remember that the success of any military mission abroad will likely depend upon the degree of domestic support demonstrated during the initial deployment and sustained operation of U.S. forces.  A clear, well-conceived, effective, and timely articulation of the legal basis for a particular mission is essential to sustaining support at home and gaining acceptance abroad.

B.    Article 2(4):  The General Prohibition against the Use of Force

1.    The UN Charter mandates that all member States resolve their international disputes peacefully.[2]  It also requires that States refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State.[3]  The majority of international law experts agree that “use of force” refers to armed force, in contrast to other forms of coercion such as economic sanctions.  This ban on aggression, taken from Article 2(4) of the UN Charter, is regarded as the heart of the UN Charter and the basic rule of contemporary public international law.[4]  An integral aspect of Article 2(4) is the principle of non-intervention, which provides that States must refrain from interference in other States’ internal affairs.[5]  Put simply, non-intervention stands for the proposition that States must respect each other’s sovereignty.

2.    U.S. policy statements have frequently affirmed the principle of non-intervention, which itself has been made an integral part of U.S. law through the ratification of the Charters of the United Nations and the Organization of American States (OAS),[6] as well as other multilateral international agreements which specifically incorporate nonintervention as a basis for mutual cooperation.  The emerging concept of humanitarian intervention,  however, has placed pressure on the principle of non-intervention and respect for State sovereignty in circumstances when a State is unable or unwilling to avert a humanitarian catastrophe, or is itself responsible for massive violations of human rights against its citizens.[7]

II.  The Lawful Use of Force

Despite the UN Charter’s broad legal prohibitions against the use of force and other forms of intervention, specific exceptions exist to justify a State’s recourse to the use of force or armed intervention.  While States have made numerous claims, using a wide variety of legal bases to justify uses of force, it is generally agreed that there are only two exceptions to the Article 2(4) ban on the threat or use of force:  (1) actions authorized by the UN Security Council under Chapter VII of the UN Charter, and (2) actions that constitute a legitimate act of individual or collective self-defense pursuant to Article 51 of the UN Charter and/or customary international law (CIL).[8]

A.   UN Enforcement Action (Chapter VII)

1.    The UN Security Council.  The UN Charter gives the UN Security Council both a powerful role in determining the existence of an illegal threat or use of force and wide discretion in mandating or authorizing a response to such a threat or use of force (enforcement).  The unique role is grounded primarily in Chapter VII of the UN Charter, which demonstrates the Charter’s strong preference for collective responses to the illegal use of force over unilateral actions in self-defense.  Chapter V of the UN Charter establishes the composition and powers of the Security Council.  The Security Council includes five permanent members (China, France, Russia, the United Kingdom, and the United States) and ten non-permanent, rotating, elected members.  Decisions within the Security Council require nine votes, and in those cases involving a non-procedural (i.e., substantive) matter, they also require the concurring votes of all five permanent members.[9]  In practice, anything other than a veto by one of the five permanent members is considered a concurring vote.  Article 24 states that UN members “confer on the Security Council primary responsibility for the maintenance of international peace and security” and, in Article 25, members “agree to accept and carry out the decisions of the Security Council in accordance with the present Charter.”

2.    Chapter VII of the UN Charter, entitled “Action With Respect to Threats to the Peace, Breaches of the Peace, and Acts of Aggression,” gives the UN Security Council authority to identify and label illegal threats and uses of force.  The Security Council may then determine what measures should be employed to address the illegal behavior.  Before acting, the Security Council must first, in accordance with Article 39, determine the existence of a threat to the peace, a breach of the peace, or an act of aggression.  Provided the Security Council makes such a determination, the UN Charter makes available several courses of action to the Security Council:  1) make recommendations pursuant to Article 39; 2) call upon the parties involved to comply with provisional measures pursuant to Article 40; 3) mandate non-military measures (i.e., diplomatic and economic sanctions) pursuant to Article 41; or 4) authorize military enforcement measures (“action by air, land, or sea forces”) pursuant to Article 42.

a.    Article 39, the same article through which the Security Council performs its “labeling” function, allows the Council to make non-binding recommendations to maintain or restore international peace and security. 

b.    Article 40 serves essentially a preliminary injunction function.  The Security Council may call upon the parties to cease action or take some action with respect to the dispute, but the parties’ compliance with those provisions will not prejudice the claims of the state in later dispute resolution proceedings.  Failure to comply with Article 40 measures may have deleterious effects for later claims.  The purpose of this Article is to prevent the aggravation of the situation that is causing a threat to international peace and security. 

c.    Article 41 lists several non-military enforcement measures designed to restore international peace and security.  These include “complete or partial interruption of economic relations and of rail, sea, air, postal, telegraphic, radio, and other means of communication, and the severance of diplomatic relations.”  Article 41 measures are stated as a mandate, binding on all UN members.  Article 42 implies that Article 41 measures must be attempted (or at least considered) before the Security Council adopts any of the military measures available to it.

d.    Article 42 contemplated that the Security Council would be able to mandate military action by forces made available to it under special agreements with UN member States under article 43.  However, because no Article 43 special agreement has ever been made, Article 42 has not operated as envisioned.  This means that the Security Council is unable to mandate military enforcement action in response to illegal threats or uses of force.  Consequently, military measures taken pursuant to Chapter VII are fundamentally permissive, phrased by the Security Council in their resolutions as an authorization rather than a mandate.

3.    In the absence of special agreements between member States and the Security Council, UN peacekeeping operations enable the Security Council to carry out limited enforcement actions through member States on an ad hoc, voluntary basis.  While these operations were traditionally grounded in Chapter VI of the UN Charter, which deals with peaceful means of settling disputes, today more peacekeeping operations are considered peace enforcement operations and carry with them a Chapter VII authorization from the Security Council.  The authorization that accompanies these operations is usually narrowly worded to accomplish the specific objective of the peacekeeping operation.  For example, UN Security Council Resolution (UNSCR) 794 (1992) authorized member States to use “all necessary means to establish, as soon as possible, a secure environment for humanitarian relief operations in Somalia.”

B.    Regional Organization Enforcement Actions.  Chapter VIII of the UN Charter recognizes the existence of regional arrangements among States that deal with such matters relating to the maintenance of international peace and security, as are appropriate for regional actions (Article 52).  Regional organizations, such as the OAS, the African Union, and the Arab League, attempt to resolve regional disputes peacefully, before referral to the UN Security Council.  Regional organizations do not, however, have the ability to unilaterally authorize the use of force (Article 53).  Rather, the Security Council may utilize the regional organization to carry out Security Council enforcement actions.  In other words, regional organizations are subject to the same limitation on the use of force as are individual States, with the same two exceptions to the general prohibition against the use of force (i.e., enforcement actions under Chapter VII, and actions in individual or collective self-defense under Article 51 of the UN Charter or CIL).  The inclusion of the International Security Assistance Force in UNSCR 1386 (2001), is an example of the Security Council relying on its power pursuant to Article 53 to authorize military action by regional organization in Afghanistan.  

III.  Self-Defense

A.   Generally

1.    The right of all nations to defend themselves was well-established in CIL prior to adoption of the UN Charter.[10]  Article 51 of the Charter provides:  “Nothing in the present Charter shall impair the inherent right of individual or collective self-defense if an armed attack occurs against a member of the United Nations until the Security Council has taken measures necessary to maintain international peace and security.”

2.    The questions that inevitably arise in conjunction with the UN Charter’s “codified” right of self-defense involve the scope of authority found therein.  Does this right, as the language of Article 51 suggests, exist only after a State has suffered an “armed attack[11],” and then only until the Security Council takes effective action?  Did the UN Charter thus limit the customary right of self-defense in such a way that eliminated the customary concept of anticipatory self-defense (see infra) and extinguished a State’s authority to act independently of the Security Council in the exercise of self-defense? 

3.    Those in the international community who advocate a restrictive approach in the interpretation of the UN Charter—and in the exercise of self-defense—argue that reliance upon customary concepts of self-defense, to include anticipatory self-defense, is inconsistent with the clear language of Article 51 and counterproductive to the UN goal of peaceful resolution of disputes and protection of international order.

4.    In contrast a majority of States, including the United States, argue that an expansive interpretation of the UN Charter is more appropriate, contending that the customary law right of self-defense (including anticipatory self-defense) is an inherent right of a sovereign State that was not “negotiated” away under the Charter.  Arguing that contemporary experience has demonstrated the inability of the Security Council to deal effectively with acts and threats of aggression, these States argue that, rather than artificially limiting a State’s right of self-defense, it is better to conform to historically accepted criteria for the lawful use of force, including circumstances which exist outside the “four corners” of the Charter.  Also note that the UN Charter, and the majority of international law experts agree that the threshold for “armed attack” is higher than that for “use of force.”  Put another way, a state could conceivably launch an operation that qualified as a use of force but fell below the standard needed for armed attack.   Thus, a so-called “gap” exists between the two terms.  The size of this gap is unclear.  Some writers such as Yoram Dinstein in War, Aggression, and Self-Defense, argue that this gap is “but a hiatus.”  The United States adopts the position that the inherent right of self-defense may exist against any illegal use of force whereas others may “draw more of a distinction” between what constitutes “armed attacks” and “uses of force that do not give rise to the right to use force in self-defense.”[12] 

B.    Self-Defense Criteria:  Necessity and Proportionality

1.    It is well-accepted that the UN Charter provides the essential framework of authority for the use of force, effectively defining the foundations for modern jus ad bellum.  Inherent in modern jus ad bellum is the customary requirement that all uses of force satisfy both the necessity and proportionality criteria.[13]

2.    Necessity.  To comply with the necessity criterion, States must consider the exhaustion or ineffectiveness of peaceful means of resolution, the nature of coercion applied by the aggressor State, the objectives of each party, and the likelihood of effective community intervention.  In other words, force should be viewed as a “last resort.”

3.    Proportionality.  To comply with the proportionality criterion, States must limit the magnitude, scope, and duration of any use of force to that level of force which is reasonably necessary to counter a threat or attack.  In the context of jus ad bellum, proportionality is sometimes referred to as “proportionate force.”  However, the principle does not require limiting the response to mirror the type of force constituting the threat or attack.[14]

C.    Types of Self-Defense

1.    Individual Self-Defense.  Within the bounds of both the UN Charter and customary practice, the inherent right of self-defense has primarily found expression in three recurring areas:  1) protection of a nation’s territorial integrity; 2) protection of a nation’s political independence; and 3) protection of nationals and their property located abroad. 

a.    Protection of Territorial Integrity.  States possess an inherent right to protect their national borders, airspace, and territorial seas.  No nation has the right to violate another nation’s territorial integrity, and force may be used to preserve that integrity consistent with the Article 51 (and customary) right of self-defense.

b.    Protection of Political Independence.  A State’s political independence is a direct attribute of sovereignty, and includes the right to select a particular form of government and its officers, the right to enter into treaties, and the right to maintain diplomatic relations with the world community.  The rights of sovereignty or political independence also include the freedom to engage in trade and other economic activity.  Consistent with the principles of the UN Charter and CIL, each State has the duty to respect the political independence of every other State.  Accordingly, force may be used to protect a State’s political independence when it is threatened and all other avenues of peaceful redress have been exhausted.

c.    Protection of Nationals.  Customarily, a State has been afforded the right to protect its citizens abroad if their lives are placed in jeopardy and the host State is either unable or unwilling to protect them.  This right is cited as the justification for non-combatant evacuation operations (NEO), discussed in greater detail later in a separate chapter of this handbook.

(1)  The protection of U.S. nationals was identified as one of the legal bases justifying U.S. military intervention in both Grenada and Panama.  However, in both cases, the United States emphasized that protection of U.S. nationals, standing alone, did not necessarily provide the legal basis for the full range of U.S. activities undertaken in those countries.  Thus, while intervention for the purpose of protecting nationals is a valid and essential element in certain uses of force, it cannot serve as an independent basis for continued U.S. military presence in another country after the mission of safeguarding U.S. nationals has been accomplished.

(2)  The right to use force to protect citizens abroad also extends to those situations in which a host State is an active participant in the activities posing a threat to another State’s citizens (e.g. the government of Iran’s participation in the hostage-taking of U.S. embassy personnel in that country in 1979-81; and Ugandan President Idi Amin’s support of terrorists who kidnapped Israeli nationals and held them at the airport in Entebbe in 1976).

2.    Collective Self-Defense.  Also referred to in Article 51, the inherent right of collective self-defense allows victim States to receive assistance from other States in responding to and repelling an armed attack.  To constitute a legitimate act of collective self-defense, all conditions for the exercise of an individual State’s right of self-defense must be met, along with the additional requirement that the victim State must consent to the assistance.  There is no recognized right of a third-party State to unilaterally intervene in internal conflicts where the issue in question is one of a group’s right to self-determination and there is no consent given by the de jure government for assistance.

a.    Collective defense treaties, such as that of the North Atlantic Treaty Organization (NATO), the Inter-American Treaty of Reciprocal Assistance (the Rio Treaty), the Security Treaty Between Australia, New Zealand, and the United States (ANZUS), and other similar agreements, do not provide an international legal basis for the use of U.S. force abroad, per se.  Such agreements simply establish a commitment among the parties to engage in “collective self-defense” as required by specified situations and provide the framework through which such measures are to be taken.  From an international law perspective, a legal basis for engaging in measures involving the use of military force abroad must still be established from other sources of international law extrinsic to these collective defense treaties (i.e., there still must be a justifiable need for collective self-defense or a UN Security Council authorization to use force).

b.    The United States has entered into bilateral military assistance agreements with numerous countries around the world.  These are not defense agreements, and thus impose no commitment on the part of the United States to come to the defense of the other signatory State in any given situation.  Moreover, such agreements, like collective defense treaties, also provide no intrinsic legal basis for the use of military force.

3.    Anticipatory Self-Defense.  As discussed above, some States embrace an interpretation of the UN Charter that extends beyond the black letter language of Article 51, under the CIL principle of anticipatory self-defense.  Anticipatory self-defense justifies using force in anticipation of an imminent armed attack.  Under this concept, a State is not required to absorb the first hit before it can resort to the use of force in self-defense to repel an imminent attack.

a.    Anticipatory self-defense finds its roots in the 1837 Caroline case[15] and subsequent correspondence between then-U.S. Secretary of State Daniel Webster and his British Foreign Office counterpart Lord Ashburton.  Secretary Webster posited that a State need not suffer an actual armed attack before taking defensive action, but may engage in anticipatory self-defense if the circumstances leading to the use of force are “instantaneous, overwhelming, and leaving no choice of means and no moment for deliberation.”  As with any form of self-defense, the principles of necessity and proportionality serve to bind the actions of the offended State.

b.    The invocation of anticipatory self-defense is fact-specific in nature, and therefore appears to lack defined standards of application, therefore it remains controversial in the international community.  Concerns over extension of anticipatory self-defense as a pretext for reprisal or preventive actions (i.e., the use of force before the coalescence of an actual threat) have not been allayed by contemporary use.  It is important to note, however, that anticipatory self-defense serves as a foundational element in the Chairman of the Joint Chiefs of Staff’s Standing Rules of Engagement (CJCS SROE), as embodied in the concept of hostile intent, which makes it clear to commanders that they do not, and should not, have to absorb the first hit before their right and obligation to exercise self-defense arises.[16]

c.    Preemptive Use of Force.  In the 2002 National Security Strategy (NSS), the United States took a step toward what some view as a significant expansion of use of force doctrine from anticipatory self-defense to preemption.[17]  This position was reinforced in the 2006 NSS, which reaffirmed the doctrine of preemptive self-defense against “rogue states and terrorists” who pose a threat to the United States based on their expressed desire to acquire and use weapons of mass destruction.[18]  The “Bush Doctrine” of preemption re-casted the right of anticipatory self-defense based on a different understanding of imminence.  Thus, the NSS stated, “We must adapt the concept of imminent threat to the capabilities and objectives of today’s adversaries.”  It concluded: “The greater the threat, the greater is the risk of inaction – and the more compelling the case for taking anticipatory action to defend ourselves, even if uncertainty remains as to the time and place of the enemy’s attack.”[19] 

d.    The U.S. analyzes a variety of factors when determining whether an armed attack is imminent.[20]  These factors include “the nature and immediacy of the threat; the probability of an attack; whether the anticipated attack is part of a concerted pattern of continuing armed activity; the likely scale of the attack and the injury, loss, or damage likely to result therefrom in the absence of mitigating action; and the likelihood that there will be other opportunities to undertake effective action in self-defense that may be expected to cause less serious collateral injury, loss, or damage.”[21]  It is the U.S. position that, “the absence of specific evidence of where an attack will take place or of the precise nature of an attack does not preclude a conclusion that an armed attack is imminent for purposes of the exercise of the right of self-defense, provided that there is a reasonable and objective basis for concluding that an armed attack is imminent.”[22] 

e.    Anticipatory self-defense, whether labeled anticipatory or preemptive, must be distinguished from preventive self-defense.  Preventive self-defense—employed to counter non-imminent threats—is illegal under international law.

D.   Self-Defense against Non-State Actors.   

1.    Many States have a greater fear of armed attacks launched by non-state actors vice attacks launched by States.  The law responsive to non-state actor armed attacks is unsettled, but the United States has stated that Article 51’s inherent right of self-defense applies to any “armed attack,” not just attacks from State actors.[23]   Many scholars base the legality of cross border attacks against non-state actors on whether the host State is unwilling or unable to deal with the non-state actors who are launching armed attacks from within its territory.[24]  Some scholars have posited that a cross-border response into a host State requires the victim State to meet a higher burden of proof in demonstrating the criteria that establish the legality of a State’s use of force in self-defense.[25]  Several countries, including the United States, have cited the unwilling and unable standard as justification for use of force in Syria in self-defense against ISIS.[26]

2.    In the wake of the attacks on the World Trade Center on 11 September 2001, the UN Security Council passed UNSCR 1368.  This resolution explicitly recognized the United States’ inherent right of individual or collective self-defense pursuant to Article 51 of the UN Charter against the terrorist actors (non-state actors) who perpetrated the 9/11 attacks.  The basis for the United States’ use of force in Afghanistan was, therefore, the Article 51 right of individual or collective self-defense.  Arguably, UNSCR 1368 signals a change where the right of self-defense against non-state actors is recognized, even if the UN Charter did not originally envision this.

IV.  Domestic Law and the Use of Force:  The War Powers Resolution

A.   In addition to the requirement that a use of force have an international legal basis, there must also be domestic legal support.  In every situation involving the possible use of U.S. forces abroad, a legal determination that embraces U.S. Constitutional principles and the 1973 War Powers Resolution must be made.[27]

B.    The Constitution divides the power to wage war between the Executive and Legislative branches of government.  Under Article I, Congress holds the power to declare war; to raise and support armies; to provide and maintain a navy; and to make all laws necessary and proper for carrying out those responsibilities.  Balancing that legislative empowerment, Article II vests the Executive power in the President and makes him the Commander-in-Chief of the Armed Forces.  This bifurcation of the war powers created an area in which the coordinate political branches of government exercise concurrent authority over decisions relating to the use of Armed Forces overseas as an instrument of U.S. foreign policy.

C.    Until 1973, a pattern of Executive initiative, Congressional acquiescence, and Judicial deference combined to give the President primacy in decisions to employ U.S. forces.  In order to reverse the creeping expansion of Presidential authority and to reassert its status as a full partner in decisions relating to the use of U.S. forces overseas, Congress passed, over Presidential veto, the War Powers Resolution (WPR).  The stated purpose of the WPR is to ensure the “collective judgment” of both the Executive and Legislative branches, in order to commit to the deployment of U.S. forces, by requiring consultation of and reports to Congress in any of the following circumstances:  1) Introduction of troops into actual hostilities; 2) Introduction of troops, equipped for combat, into a foreign country; or 3) Greatly enlarging the number of troops, equipped for combat, in a foreign country.

D.   The President is required to make such reports within forty-eight hours of the triggering event, detailing the circumstances necessitating introduction or enlargement of troops, the Constitutional or legislative authority upon which the action is based, and the estimated scope and duration of the deployment or combat action.

E.    The issuance of such a report, or a demand by Congress for the President to issue such a report, triggers a sixty-day clock.  If Congress does not declare war, specifically authorize the deployment/combat action, or authorize an extension of the WPR time limit during that period, the President is required to withdraw deployed forces.  The President may extend the deployment for up to thirty days, should the circumstances so require, or for an indeterminate period if Congress has been unable to meet due to an attack upon the United States.

F.    Because the WPR was enacted over the President’s veto, one of the original purposes of the statute—establishment of a consensual, inter-branch procedure for committing our forces overseas—was undercut.  Although the applicability of the WPR to specific operations will not be made at the Corps or Division level, once U.S. forces are committed overseas, a deploying JA must be sensitive to the impact of the WPR on the scope of operations, particularly with respect to the time limitation placed upon deployment under independent Presidential action (i.e., the WPR’s 60-90 day clock).

G.   Procedures have been established which provide for CJCS review of all deployments that may implicate the WPR.  The Chairman’s Legal Advisor, upon reviewing a proposed force deployment, is required to provide to the DoD General Counsel his analysis of the WPR’s application.  If the DoD General Counsel makes a determination that the situation merits further inter-agency discussion, he or she will consult with both the State Department Legal Adviser and the Attorney General.  As a result of these discussions, advice will then be provided to the President concerning the consultation and reporting requirements of the WPR.