The Use of Force Against ISIS Commentary
The Use of Force Against ISIS
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JURIST Guest Columnist Bilal Khan discusses the legal implications of the use of force against ISIS in IRAQ…

The Factual Background

The Islamic State in Iraq and the Levant (ISIS) made headlines in June, when it escalated its offensives in Iraq. By June 10, it had captured Iraq’s second largest city, Mosul, followed by Tikrit, Rawa, Ana and Al-Qaim. It now controls border crossings with Syria, and pledges to march on Baghdad. Its aim: to forcibly establish a state across the region.

ISIS has a stronghold in Syria, where it controls large areas including eastern Aleppo and Raqqa, as well as oil and gas fields. With assets worth an alleged $2 billion, ISIS has little difficulty recruiting personnel or obtaining weaponry.

Iraq has taken steps to combat ISIS within its own territory, with limited US assistance. Talk abounds of the prospect of US-led military strikes across the border. When policy makers consider their options, it is essential to consider this question: how are the international legal rules governing the use of force to be applied to military action against ISIS in Iraq and/or Syria.

The Prohibition of Force

The heart of the rules on the use of force is Article 2(4)
 of the United Nations Charter. This prohibits the “threat or use of force against the territorial integrity or political independence of any State”. Recognized as jus cogens, states cannot derogate from Article 2(4). The only widely accepted exceptions to the prohibition are Chapter VII action by the Security Council and the right of self-defense (this article considers the latter).

Before looking at exceptions, a logically prior question is whether intervention in Iraq at the request of the Iraqi Government would engage Article 2(4) at all. Iraq as a sovereign State has the right to ask others for assistance in order to repel non-state actors (NSAs) within its own territory. Such action will not offend Iraq’s territorial integrity or political independence. Article 20 of the ILC’s Articles on State Responsibility 
[PDF] provides: “Valid consent by a State to the commission of a given act by another State precludes the wrongfulness of that act … to the extent that the act remains within the limits of that consent”.

That provides a legal basis for the use of force within Iraqi territory, but only if one assumes a purely internal armed conflict. A significant locus of the ISIS threat and source of current attacks is arguably Syria. What if the battle is taken across the border? The use of force on Syrian territory without the Syrian Government’s consent will prima facie infringe its territorial integrity and entail a breach of Article 2(4). The only justification for such action would be self-defense, which in the writer’s view affords a more appropriate characterization for Iraqi and allied actions both inside and outside Iraq.

The Law of self-defense

Article 51 of the UN Charter provides:

“Nothing in the present Charter shall impair the inherent right of individual and collective self-defense if an armed attack occurs…”

Article 51 was not intended to delineate the scope of self-defense, but rather to affirm its continuation post-1945 and safeguard regional security arrangements. Further, as confirmed by the ICJ, the content of self-defense is influenced by customary international law. Custom, in turn, depends on evolving State practice, and so the right is not static.

Today, there are three requirements:

1. An actual or imminent armed attack against a State;

2. The attack must attain a minimum scale;

3. Necessity and proportionality of the armed response.

Is there an “Armed Attack”

The first question is whether Iraq has suffered an armed attack. This in turn revives the controversy over whether NSAs can in law commit armed attacks, justifying a forcible response on foreign soil. For some, the Caroline case of 1837
 supports this proposition. A ship, the Caroline, had been used in a rebellion against British rule in Canada, but there was no question of United States’ involvement. British forces seized and destroyed it while it was moored in New York State.

The difficulty with this argument is that up until recently, State practice unequivocally held that attacks had to be attributable to a State. In 1985 Israel took action against the PLO 
based in Tunis on the basis that Tunisia had “harbored the PLO.” The action was condemned in Security Council Resolution 573 as the threshold for attribution was not met. In 1995 the international community condemned Turkey’s actions against the PKK 
in northern Iraq. Similarly in Nicaragua the ICJ held that an attack by rebels would only be an “armed attack” if they had been sent “by or on behalf” of a State.

The law has moved on. Today, an armed attack can be committed by NSAs without any State involvement. This was put conclusively beyond doubt by the welcoming international response to the operations against Al-Qaeda in Afghanistan in 1998 and 2001. Security Council resolutions 1368 and 1373 
recognized the “inherent right of self-defense” in the context of terrorism, without any suggestion of attribution to a State. Even during Israel’s offensive in Lebanon in 2006, few States in the Security Council debates questioned Israel’s right to respond to rocket attacks and kidnapping; the disagreement was instead over proportionality. In each case, however, the host State had violated its international obligations 
by “tolerating” armed NSAs on its territory.

Accordingly, nothing turns on the fact that ISIS’ attacks cannot be attributed to a State. Prima facie its attacks constitute “armed attacks” under Article 51, so long as the other requirements are met.


According to Nicaragua an attack must attain a minimum scale in order to qualify as an armed attack. The rationale is to exclude minor disturbances. In similar vein Dinstein
 argues that an “armed attack presupposes a use of force producing serious consequences, epitomized by territorial incursions, human casualties or considerable destruction of property.” In Oil Platforms
 [PDF] however, the ICJ refused to exclude the possibility that the mining of a single military vessel would meet the requirement.

On any interpretation, ISIS’ attacks are of sufficient scale. It has forcibly taken Iraqi towns and cities, and attacked armed forces and civilians. Although framed with inter-state warfare in mind, its attacks fall within the spirit of Article 3 of the UN Definition of Aggression.

An Actual or Imminent Attack

As stated above, an armed attack must be under way or imminent. In Iraq today the ISIS attacks are ongoing. Further, given the intelligence position made public in conjunction with the stated future aims of the organization and its capability, there is an imminent threat of further attacks.

The Extra-territorial Requirement

It must be shown that ISIS’ attacks are not purely internal. Put differently, there must be an international dimension. The ICJ in its Wall Advisory Opinion 
held that “the threat which [Israel faces] originates within, and not outside, that territory…and therefore Israel could not … invoke a right of self-defense”.

In the writer’s view ISIS’ attacks are sufficiently internationalized. Up until recently, the preponderance of its activity was in Syria, where it controls large areas of territory, natural resources and weaponry. Moreover, it appears that ISIS personnel and weaponry are crossing the border into Iraq. Indeed, it has seized border crossings between the two States for this purpose. The organization also has broad international aims. The threat, therefore, does not originate purely within Iraqi territory. To argue the contrary would be to unnecessarily undermine Iraq’s right to defend itself.

Necessity and Proportionality

Even if ISIS has committed armed attacks it must still be necessary for Iraq and its allies to use force. As Roberto Ago states in his 8th Report to the ILC on State Responsibility 
[PDF] “the State must not have had any means of halting, repelling or preventing the attack other than recourse to armed force.” In the current case, necessity flows automatically from the fact that an attack is underway. Moreover, an attempt to resolve the issue peacefully would enable ISIS to consolidate its gains and prepare further attacks.

None of this means that it is permissible to use force on Syrian territory per se. Post 9/11 State practice suggests that self-defense against NSAs is permissible when the host state has acquiesced in attacks contrary to international law. An example would be Afghanistan’s toleration of Al-Qaeda. Syria conversely has not acquiesced in rebel activities; it rather lacks the capability to dismantle ISIS, which of itself is not unlawful. As a result consent should be sought from the Syrian Government before taking action on its territory. Given Syria’s inability to tackle ISIS itself, any refusal will be unreasonable and cement the necessity for action.

Any defensive response must be proportionate. This has two elements. Firstly, the response must not be manifestly out of proportion to the scale of ISIS’ attacks. This does not, however, require complete symmetry of scale between the two; in 1982 the UK had to use significantly more force in recapturing the Falklands than Argentina had used in taking it.

Secondly, the objective of the action must be reasonable, and the action itself must not go beyond what is necessary to achieve the objective.

Air strikes against ISIS bases, assets and personnel within Syrian territory will arguably be proportionate to both the scale of ISIS’ attacks, and the reasonable aim of diminishing its capability.


Iraq has a right to self-defense against armed attacks perpetrated by ISIS. There are advantages to characterizing the attacks as armed attacks, since it prima facie renders it permissible to take the battle to Syrian territory. The strict requirements of necessity and proportionality limit the scope of the ensuing response.

Iraq would be entitled to call for assistance from its allies, such as the US, transforming the action into collective self-defense. Defensive actions must be reported to the Security Council as required by Article 51.

Bilal Khan is a civil litigation lawyer in the United Kingdom’s government sector. Prior to this he assisted human rights organizations and the British Institute of International and Comparative Law. He has an LLM (Distinction) from the London School of Economics, where he specialized in Public International Law. The views expressed here are his own.

Suggested citation: Bilal Khan, The Use of Force Against ISIS, JURIST-Hotline, July 3, 2014,

This article was prepared for publication by Jason Kellam, a Section Editor with JURIST’s Commentary services. Please direct any questions or comments to him at

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