Striking ISIL: Aspects of the Law on the Use of Force

Marc Weller
March 11, 2015

On September 20, 2014, the government of Iraq informed the UN Security Council (SC) that it had requested the United States to lead international efforts to strike Islamic State in Iraq and the Levant (ISIL) sites. The strikes would end the constant threat posed by ISIL to Iraq, protect Iraq’s citizens, and ultimately enable Iraq to regain control of its borders.[1] The U.S. asserted that this request extended to ISIL sites in Syria.[2]

This Insight focuses on two aspects of this development. The first concerns the right of the government of Iraq, deprived of control over significant parts of Iraqi territory, to receive international assistance. The second concerns the reach of such assistance beyond the borders of Iraq, to Syria, in application of the right to collective self-defense.

Authority to Represent

When requesting U.S. military assistance, the Iraqi government had already lost control over significant parts of its territory to ISIL. Indeed, with the apparent collapse of the Iraqi regular forces over last summer, ISIL appeared close to be marching on the capital, Baghdad.[3]

Traditionally, a government can continue to represent a state until it has been entirely displaced by a popular uprising or armed opposition movement—until it has lost control over virtually all territory, including the Presidential Palace, as it were. Up to that point, the government will claim the right to request and receive military assistance in its struggle to survive. Conversely, any military support given to the opposition movement would be considered as intervention.

However, recent practice has advanced upon this classical position, emphasizing legitimacy and representativeness of governance over traditional criteria favoring the established authorities.  The routine response of the SC to protracted internal conflict leading to the loss of control over significant portions of territory and population is to deprive both sides, government and opposition, equally of external military support through an arms embargo. The rationale is that a fundamental dissociation between the government and the governed has occurred, putting into question the right of the government to represent the state in relation to military assistance. Instead of relying on the outcome of armed contestation, the authority to govern is to be reallocated to the true sovereign, the population, rather than the armed groups. Invariably this would then be pursued through a UN sponsored peace process, often including a constitutional review and fresh elections.

While this advanced practice has been applied in a broad range of cases, there are exceptions. In fact, Syria is the most notorious instance where a blockage in the SC precluded this path. A U.K. draft resolution threatening an arms embargo covering both sides under Article 41 of the UN Charter failed due to a combined Russian Federation and Chinese veto.[4]

The government in Damascus has been sustained by supplies of advanced weapons and ammunition, mainly supplied by the Russian Federation.[5] Iran has been deploying proxy forces through Hezbollah and its regular al Quds special forces—ironically the same division which is now also said to be active against ISIL in Iraq.[6] Other states have supplied the armed opposition, whose ranks have also been swelled by foreign fighters.

The West has strongly criticized those supporting the regime of Bashar al-Assad. But if it is unlawful or at least questionable to continue armed support for the Assad regime, why would it be lawful to do the same in relation to the Iraqi government? Both have lost control over large swathes of territory at the hands of armed movements. Indeed, both have lost control to ISIL, although other opposition groups have also captured other portions of Syria.

The answer to this question is a qualitative one. Both Iraq and Syria have held recent elections. President al-Assad claimed an 88.7% electoral victory, with a purported participation of 73.42%.[7] However, the elections, held amidst a vicious civil war and only in areas under immediate control of the government, were not credible, as had been advised by the UN even before they were held.[8]

The parliamentary elections in Iraq, followed by the appointment of a new head of government in accordance with the electoral result, were fully accepted and welcomed by the UN Secretary-General. [9] In fact, the request for military assistance was quite deliberately delayed until a full government had been formed which represented all regional, religious and ethnic segments of the country.

By that time, ISIL had imposed itself with bizarre rapidity upon the population, seizing large parts of territory. Iraq duly reported this development to the SC over the summer, indicating that ISIL was terrorizing citizens; carrying out mass executions; persecuting minorities and women; and destroying mosques, shrines, and churches.[10] The Iraqi government remained the principal agent of the responsibility to protect its population from such outrages. Indeed, the SC urged the international community “to further strengthen and expand support for the government of Iraq as it fights ISIL and associated armed groups.”[11] The Council even endorsed the decision of the Paris Conference of September 15, 2014, where twenty-six governments offered military assistance.

Like Iraq, the government of Syria claimed to have been subjected to a massive campaign of foreign-supported terrorism. In reality, of course, the conflict was caused by the rejection of its autocratic government by broad masses of the population in the context of the Arab Spring. The military confrontation that ensued opened space for terrorist groups, such as al-Nusra and ISIL, to establish themselves. However, it was the opposition which took up the fight against these groups, rather than the government of Syria, which profited by this diversion of the energy.

Hence, the Syrian government’s legitimacy deficit was twofold. It consisted of its active and brutal campaign against its own population, and its passive toleration of the establishment of ISIL on its territory for many months, failing in its responsibility to protect on both counts.

The SC was unable to express this negative differentiation between Syria and Iraq, given the application of the veto to the former. Nevertheless, the differentiation was made, positively, in its endorsement of international support for Iraq, which is obviously lacking in relation to Syria. Hence, even in this difficult political environment, subjective considerations of legitimacy displaced a mechanical application of the principle of effectiveness in determining governmental authority to represent.

External Use of Force

The Iraqi letter to the SC requesting international support refers to the fact that ISIL “has secured for itself the ability to train for, plan, finance and carry out terrorist operations across our borders. The presence of this safe-haven has made our borders impossible to defend and exposed our citizens to the threat of terrorist attack.”[12]

This language might be suggestive of armed intervention, rather than armed attack. However, the SC Statement following the letter finds that a “large scale offensive” by ISIL is taking place. [13] This confirms that the campaign is of sufficient military character, intensity, and breadth to qualify as an armed attack triggering self-defense under Article 51 of the Charter.

A non-state entity can mount an “armed attack” that triggers self-defense, notwithstanding the confused reading often given to the Advisory Opinion in the Wall case by the International Court of Justice (ICJ).[14] The original determination to this effect in SC resolutions 1368 and 1373 of September 2001 is now confirmed by this SC Statement which condemns the “attacks” by ISIL and associated groups.[15]

But does self-defense extend to striking ISIL bases in Syria, thus potentially also injuring the rights of the territorial state? This issue has given rise to debate about “attribution” of an armed attack mounted by ISIL from Syria against Iraq. Under the ICJ’s Nicaragua test, it is argued that this would require “direction” or “effective control” by Syria over the specific military operations conducted by ISIL.[16] Evidently, this standard, or even the somewhat lower test of “overall control” over the non-state organization in question proposed by the ICTY in Tadic, is not met in this case.[17] Instead of being directed by Syria, ISIL has displaced Syrian authorities in Syria as much as ISIL has displaced Iraqi authorities in Iraq.

In the alternative, the U.S. report to the Council claims that self-defence applies where the “government of the state where the threat is located is unwilling or unable to prevent the use of its territory for such attacks. The Syrian regime has shown that it cannot and will not confront these safe havens effectively itself.”[18] The UN Secretary-General appeared to offer an implicit endorsement when he noted that the U.S. strikes “took place in areas no longer under the effective control of the government.”[19]

The doctrine of unwilling or unable remains subject to some debate, as it is seen as a new legal justification for the use of force which would detract from the jus cogens rule of the prohibition of the use of force.[20] In reality, this debate, along with the discussion about attribution, is misguided. Self-defense in the sense of Article 51 of the Charter is a provisional right to preserve the state from an actual or imminent attack. Such an actual or imminent attack is in itself sufficient to trigger the right of self-defense against its source. If, on the other hand, there is no actual or imminent use of force amounting to an armed attack, the doctrine of “unwilling or unable” cannot furnish a legal justification for the use of force in self-defense. Instead, in the absence of an instant and overwhelming necessity of action, leaving no choice of means and no moment of deliberation, non-forcible remedies would need to be applied.

Self-defense applies in view of the immediacy of the situation and the necessity and proportionality of an armed response. This already implies that the territorial state would not forestall or terminate the imminent threat or attack—if it did, the criterion of necessity would not be met. Hence, there is no additional need to prove that the territorial state is unwilling or unable to act.

Similarly, if the threat or attack is actual or imminent, it is not necessary to demonstrate that it is attributable to the territorial state from which it emanates. Attribution is a matter of the law on state responsibility, rather than the jus ad bellum. Both are distinct concepts and operate according to distinct criteria.[21] In this instance, the infrastructure of ISIL in Syria is sufficiently closely intertwined with its ongoing operations in Iraq to justify extending the application of the right to self-defense to Syria.

About the Author: Marc Weller, an ASIL member, is Professor of International Law and International Constitutional Studies in the University of Cambridge and Director of the Lauterpacht Centre for International Law. He is the editor of the Oxford Handbook on the Use of Force in International Law. The author gratefully acknowledges the assistance and suggestions provided by Mr. Jake Rylatt.


[1] Permanent Rep. of Iraq to the U.N., Letter dated Sept. 20, 2014 from the Permanent Representative of Iraq to the United Nations addressed to the President of the Security Council, U.N. Doc. S/2014/691, (Sept. 22, 2014) [hereinafter Letter from the Permanent Rep. of Iraq].

[2] Permanent Rep. of U.S. to the U.N., Letter dated Sept. 23, 2014 from the Permanent Representative of the United States of America to the United Nations addressed to the Secretary-General, U.N. Doc. S/2014/695, (Sept. 23, 2014) [hereinafter Letter from the Permanent Rep. of U.S.].

[3] Extremists in Iraq Continue to March Toward Baghdad, Time, June 11, 2014, available at

[4] S.C. Draft Res. 538, U.N. Doc. S/2012/538 (July 19, 2012).

[5] Emile Hokayem, Syria – Assad Regime Maintains Military Superiority, IISS Military Balance Blog (Feb. 9, 2015),

[6] Id.

[7] Bashar al-Assad Wins Re-election in Syria as Uprising Against him Rages on, The Guardian, June 4, 2014, available at

[8] UN Cautions against Holding Presidential Election Amidst Ongoing “Tragedy”, UN News Centre, Apr. 21, 2014, available at

[9] UN Chief Lauds Formation of New Government in Iraq,’ UN News Centre, Sept. 9, 2014, available at

[10] Permanent Rep. of Iraq to the U.N., Letter dated 25 June 2014 from the Permanent Representative of Iraq to the United Nations addressed to the Secretary-General, U.N. Doc. S/2014/440 (June 25, 2014).

[11] U.N. President of the S.C., Statement by the President of the Security Council, U.N. Doc. S/PRST/2014/20, (Sept. 19, 2014) [hereinafter Statement by the President of the SC].

[12] Letter from the Permanent Rep. of Iraq, supra note 1.

[13] Statement by the President of the SC, supra note 11.

[14] The ICJ did not in fact hold that activities of non-state actors cannot give rise to self-defense, but that Israel could not invoke self-defense against a non-state actor operating in territory it occupied. Legal Consequences of the Construction of Wall in the Occupied Palestinian Territories, Advisory Opinion, 2004 I.C.J. 136 (July 9).

[15] Statement by the President of the SC, supra note 11.

[16] Military and Paramilitary Activities in and Against Nicaragua (Nicar. v. U.S.),  1986 I.C.J. 14, ¶ 115 (June 27); and more clearly, Case Concerning Armed Activities on the Territory of the Congo, (Dem. Rep. Congo v. Uganda), 2005 I.C.J. 168, ¶ 146 (Dec. 19); Application of the Convention on the Prevention and Punishment of the Crime of Genocide (Bosn. & Herz. v. Serb. and Montenegro), 2007 I.C.J. 43, ¶¶ 402–407 (Feb. 26).

[17] Prosecutor v. Tadic, IT-91-1-A, Appeal Judgment, (Int’l Crim. Trib. for the Former Yugoslavia July 15, 1999).

[18] Letter from the Permanent Rep. of U.S., supra note 2.

[19] Secretary-General Ban Ki-moon, Secretary-General of the U.N., Remarks at the Climate Summit Press Conference (Including Comments on Syria) (Sept. 23, 2014), available at

[20] Jens David Ohlin, The Unwilling or Unable Doctrine Comes to Life, Opinio Juris, Sept. 23, 2014, 8:10 PM),

[21] On the dual nature of self-defense as an independent right and as a circumstance precluding wrongfulness, see Jean-Marc Thouvenin, Circumstances Precluding Wrongfulness in the ILC Articles on State Responsibility: Self-defence, in The Law of International Responsibility 455, 459 (James Crawford, Alain Pellet and Simon Olleson eds., 2010).