Anticipatory (Pre-emptive) Self-defence:

The Need for a Modern Approach


Sarah Champion

Officer Cadet


Royal Military College, Canada


The use of military force is a valid customary international law norm and it is enshrined in the United Nations Charter.  Nevertheless, the use of force is only authorised if it falls under one of two categories: self-defence (article 41 of the United Nations Charter), or Security Council authorisation.  To justify a resort to pre-emptive war, a state must give reasonable proof that the action is necessary to the vital national security interests of the state, and that the act of aggression in self-defence is proportional, according to Charter principles.  The threat imposed by an aggressor must be proven to be clear and imminent, direct, critical to the state facing disproportionate danger, and unable to be handled using peaceful alternatives.  According to the Charter, to deem self-defence lawful requires that an attack has already been launched against a victim state.  Examples of states acting pre-emptively in anticipatory self-defence have further elaborated on this legality, creating in some instances an international acceptance that in the case of an imminent attack, the necessity of a proportional assault in self-defence is lawful.  However, the issue remains that the Charter, in order to deem an action as lawful self-defence, requires the existence of an armed attack on the victim state.  Interpretation on what constitutes an armed attack is what generates the most disagreement amongst the international law community.  It is agreeable, however, that no state can be expected to sit idly by and await the first blow of an armed attack by an aggressor state in the modern era of warfare. 


The recent War on Terrorism is giving us an example of how this particular article of the Charter can be difficult to interpret, and creates a need for modernising the internationally accepted norms and evolving the Charter’s principles on self-defence in order to accommodate victim states.  

The term “War on Terrorism” itself refers to the policy made by President Bush immediately following the September 11 terrorist attacks, declaring that the U.S. would "make no distinction between the terrorists who committed these acts and those who harbour them".[1] The immediate application of this policy was the invasion of Afghanistan in early October 2001, when the Taliban controlled government of Afghanistan refused to hand over the well-known al-Qaida terrorist leader Osama bin Laden.  This new United States policy inferred that any nation refusing to cooperate with American efforts to attack terrorists would be considered an enemy state. On September 20, in a televised address, Bush summed up this policy with the words, "Every nation, in every region, now has a decision to make. Either you are with us, or you are with the terrorists."[2]  This doctrine is argued to be contrary to the classical concept of a just war which requires, among other stipulations, that war must only be conducted in self-defence. Supporters of the doctrine counter that the state-sponsorship of terrorism is in itself a first act of war, and that the US is acting justly when it answers with military actions.


The question is whether or not the existence of weapons of mass destruction (WMD) at the hands of terrorists and non-state actors changes the act of anticipatory defence to a clearly legal justification, and how the existence of such a threat to the national security of a state can be addressed.  Certainly, it must be acknowledged that in preparing defences against a nuclear attack, there is no lead time.  With this fact in mind, some experts in the field argue that a law restricting countries to act only when an act of aggression and hostility against them is underway is out-dated and not relevant to the immediacy and severity of nuclear weapons.  However, most international jurists would agree that a nation’s capacity to launch nuclear weapons is not sufficient to warrant defensive attacks against it.  Considering transnational terrorism and the evolution of the conventional battlefield, it is evident that the extremely restrictive formula introduced by Daniel Webster in 1841 does not account for the modern era of weapons and warfare, especially in the statement “leaving no choice of means and no moment for deliberation”.  International law has long recognised the right of a state to defend itself against imminent danger.  In a world today that is faced with terrorism, the concept of imminent danger should be adapted, yet this process should ensure that there exists a global body capable of monitoring the field of anticipatory self-defence.


The criterion for lawful self-defence derives from many legal backgrounds that have become the norm in international law standards.  The classic formula that supports the principle of anticipatory self-defence stems from the Caroline incident which arose in 1837.  The main reason for recalling this incident is for then U.S. Secretary of State Daniel Webster’s classic formulation of the essential elements of anticipatory self-defence: immediacy, necessity, and proportionality[3].  Because of these elements, this is the case most often cited when determining whether or not a state has the right to resort to force when faced with imminent danger to national security.  In brief, this incident took place during a rebellion of Canadians against the British Crown government. 

A band of Canadian rebels, seeking a more democratic Canada, had been forced to flee to the United States after leading the failed Upper Canada Rebellion[4].  They took refuge on Navy Island on the Canadian side of the Niagara River and declared themselves the Republic of Canada.  The rebels were supported in their cause by many Americans, who considered the rebellion a continuation of the American Revolutionary War.  They supplied the Canadian insurgents with supplies, money, provisions, and arms via the steamboat Caroline, and also enlisted to fight in an invasion of Canada.  Despite the knowledge of these occurrences, the United States government did not attempt to stop the Americans from aiding the Canadian rebels, and claimed that the United States government did not have the authority to force neutrality.  On December 29 Colonel Sir Allan MacNab ordered a party of militia to cross the river and set the Caroline ablaze. Finding her docked at Fort Schlosser, New York, they seized her, towed her into the current, set her afire and cast her adrift over Niagara Falls, killing one American in the process[5].  American forces burned British steamer Sir Robert Peel while it was in the US.  The tensions were ultimately settled by the Webster-Ashburton Treaty.  Several years after the affair, through an exchange of diplomatic messages, Webster argued that the sovereignty of the United States had been violated and that the British, to declare this act in self-defence, would have had to “show a necessity of self-defence, instant, overwhelming, leaving no choice of means, and no moment for deliberation”[6].  Furthermore, he argued that the act, if justified by self-defence necessity, must be proportional, and not exact greater means than necessary.  From this correspondence, the rules for anticipatory self-defence were formed: imminency, necessity, and proportionality. 


This criterion, which later became the norm in international law relating to anticipatory self-defence in another sovereign state’s territory, can be broken into four elements that must be fulfilled.  The first is necessity, due to a grave threat of a great magnitude.  The second element is the likelihood that the threat will be realised unless anticipatory self-defence occurs, stemming from imminency of attack.  The third element is the exhaustion of all peaceful means and alternatives to using force.  The last of these elements is that the use of pre-emptive force is consistent with Charter principles, namely proportionality. 


After the Caroline affair, both imminence and necessity became crucial factors in determining self-defence[7].  However, one hundred years following the incident, the UN Charter was adopted, which changed the scope in regards to the use of force by states.  The UN Charter prohibits all use of force, except in two circumstances: the first being with the explicit authorisation of the Security Council under Chapter VII, Article 42, and the second case being in self-defence as read in Article 51 in response to an attack on the territory of a member state.  Therefore, Article 2 (4) of the UN Charter requires countries to refrain from the use of force, but permits in Article 51 the exercise of the “inherent right of individual or collective self-defence if an armed attack occurs…until the Security Council has taken measures necessary to maintain international peace and security”[8].  Many international lawyers assert that the Charter explicitly prohibits the use of pre-emptive force until after an attack, as it is written in print in the Charter.  Consequently, there have been countless debates over the Charter’s meaning of “armed attack”.  It has been argued that this term implies a state may only act in self-defence when an attack has actually occurred, if the text is read literally.  However, in the modern era of warfare and advanced weaponry, no state can be expected to wait to be a victim of the first attack before defending the nation, if the state’s fundamental security is at stake.  The nature of threats to security has undoubtedly changed since the inception of the Charter in 1945, when the assumption was that a global security organisation could effectively control the use of force by states[9]. 


There are two major events that are cited to demonstrate the legitimacy of anticipatory self-defence, despite the adoption of the United Nations Charter.  The first of these which is often referred to for legal advice in the realm of anticipatory self-defence is Israel’s pre-emptive strike at the beginning of the 1967 Six-Day War.  Israel launched the attack on the Egyptian army which was massing at its borders: in this situation, immediacy is illustrated, with a visible line of troops and weaponry, and likewise, intent is obvious, which was furthered by intercepting communications which clearly outlined the plans of the Egyptian forces. 


The second commonly cited incident is Israel’s 1981 bombing of Iraq’s Osirak nuclear reactor, following intelligence sources indicating to the Israeli government that the Iraqis were preparing to turn the nuclear reactor on.  The Israelis decided to deal with the perceived threat sooner rather than later, and launched airstrikes, claiming that this action was vital to protect national security, and was acted out in the basis of self-defence.  However, this attack was declared an illegal preventive attack which was said to have threatened regional security, and the Security Council accordingly condemned the action, claiming that the Israelis had failed to exhaust peaceful means in order to resolve the perceived situation. 


The problem that is encountered in defining the legality of pre-emptive self-defence is found in terrorism.  The rule prohibiting the use of force applies only to states.  Terrorists are not subject to international law, as they engage themselves in non-state violence[10].  However, in 1974, the General Assembly concluded that in situations where a state is involved in non-state violence, the very involvement is equivalent to an armed attack.  Thus, a state falling victim to terrorist attacks from a country harbouring or supporting terrorists, could invoke the right of self-defence.  It was on the basis of this ruling that the United States justified the intervention of the American state and her allies in Afghanistan following the attack on the Twin Towers on September 11th. 


Although the guidelines regarding pre-emptive self-defence and the use of force are defined in the UN Charter and in international law norms, the reality is that terrorism is transnational and terrorists are not subject to the same deterrent pressures that States are.  Terrorism is a grave danger, and the difficulty comes in reaching terrorists that are being harboured and/or supported by sovereign states.  No matter how technologically advanced and prepared a military or state is, the prevention of serious terrorist attacks is not feasible.   


Such is the justification behind the new approach to terrorism that has been adopted by the Bush administration as part of the new National Security Strategy (NSS) of 17 September 2002.  Essentially, the NSS has changed the approach to terrorism from one of a criminal offence to a grave threat to national security.  Although the Strategy does mention that this new approach will not allow the US to “use force in all cases to pre-empt emerging threats, nor should nations use pre-emption as a pretext for aggression”[11], the NSS clearly asserts the United States’ right to stop terrorists and rogue states before they are able to inflict damage on the American state or her allies.  


The Bush administration has stated that “the greater the threat, the greater 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”[12].  This is a dangerous conclusion to be drawn.  Too many questions are left unanswered[13]: if the international community decides that this new doctrine of pre-emptive self-defence is applicable in cases of non-state aggression, when and on what terms will it be acceptable for one state to attack terrorists residing in another sovereign state?  Who will be responsible for drawing the lines of legality, and deciding when a terrorist threat is grave and imminent enough to justify anticipatory actions[14]?  Certainly, it is without doubt that the existence today of weapons of mass destruction in the hands of transnational actors who are not subject to the international laws on the use of force creates a need for the international community to consider an evolution of the Charter principles and exclusions to the  laws regarding the use of military force in self-defence.  If correctly applied, anticipatory or pre-emptive self-defence is part of a state’s legitimate authority to use self-defence.  However, we must be careful in evolving the international norms relating to this subject, and must be aware that if we include terrorists in the definition of an armed attack, we are striking a thin balance when it comes to state sovereignty. 


Although this paper does not attempt to criticise nor support the US-led intervention into Iraq, it goes without saying that the US actions in Operation Iraqi Freedom have been globally criticised as being unlawful.  This is due to the grey area that lies between outright unlawful aggression, waged against a nation that may someday develop a desire to attack a nation, and self-defence, which is a strike against another state during a war.  One cannot act aggressively towards another solely because the person does not particularly like them, and may someday consider hostility against them.  A policy of pre-emption and prevention, and a focus on a state’s ability to act, rather than on the actual intent of the state, is a dangerous policy.  The NSS introduced by President Bush in 2002 borders on this line:  “…even if uncertainly remains as to the time and place of the enemy’s attack” demonstrates too rapid an evolution of international law regarding the legitimate use of force by states.  There must be some way of preventing states from acting liberally behind a facade of excuses and justification on the grounds of anticipatory self-defence.  


Works Cited


Bothe, Michael.  “Terrorism and the Legality of Pre-emptive Force”.  European Journal of

International Law.  Vol. 14, No. 2 (2003): 227-240.


Cullinan, John F.  “The Preemption Doctrines”.  National Review Online.  (2002).  26 July 2004


Ratner, Steven R.  “Jus Ad Bellum and Jus In Bello after September 11”.  American Journal Of

International Law.  Vol. 96, No. 4 (2002): 905-921.


Sapiro, Miriam.  Iraq: The Shifting Sands of Pre-Emptive Self-Defence”. American Journal Of

International Law.  Vol. 97, No. 3 (2003): 599-607.


Schmitt, Michael.  “Pre-emptive Strategies and International Law”.  Michigan Journal of

International Law.  (2003): 513-448.


Sofaer, Abraham D.  “On the Necessity of Pre-emption”.  European Journal of International Law.

Vol. 14, No. 2  (2003): 209-226.


Taft, William H. and Todd Buchwald.  “Preemption, Iraq, and International Law”.  American

Journal Of International Law.  Vol. 97, No. 3 (2003): 557-563.


The Free Encyclopaedia.  US Governmental Response to the September 11 2001 Attack.



United States National Security Strategy. 17 September 2002.  United States Department of State

Information Service.  22 July 2004.





[1] The Free Dictionary.  US Governmental Response to the September 11 2001 Attack.

[2] Ibid

[3] Cullinan, John F (2002).  “The Preemption Doctrines”.  National Review Online.  Available at:

[4]  The Free Dictionary.  US Governmental Response to the September 11 2001 Attack.

[5] The Free Dictionary.  US Governmental Response to the September 11 2001 Attack.

[6] Cullinan, John F (2002).  “The Preemption Doctrines”.  National Review Online.  Available at:

[7] Sapiro, Miriam. (2003)  Iraq: The Shifting Sands of Pre-Emptive Self-Defence”. American Journal OfInternational Law.  Vol. 97, No. 3, p. 600.

[8] Sapiro, Miriam. (2003) “Iraq: The Shifting Sands of Pre-Emptive Self-Defence”. American Journal Of International Law.  Vol. 97, No. 3, p. 601.

[9] Sapiro, Miriam. (2003)  Iraq: The Shifting Sands of Pre-Emptive Self-Defence”. American Journal Of International Law.  Vol. 97, No. 3, pp. 601-602.

[10] Bothe, Michael.(2003)  “Terrorism and the Legality of Pre-emptive Force”.  European Journal of International Law.  Vol. 14, No. 2 : p. 230.

[11] United States National Security Strategy. 17 September 2002.  United States Department of State

Information Service.  Available at:

[12] Sapiro, Miriam. (2003)  Iraq: The Shifting Sands of Pre-Emptive Self-Defence”. American Journal Of International Law.  Vol. 97, No. 3, p. 602.

[13] Schmitt, Michael.(2003).  “Pre-emptive Strategies and International Law”.  Michigan Journal of

International Law. Pp 513-548.

[14] Schmitt, Michael. (2003)  “Pre-emptive Strategies and International Law”.  Michigan Journal of International Law.  Pp 513-548.