Why Trump’s drone strike on Qasem Soleimani was an illegal use of force under international law

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On 3rd January 2020, tension amid the Persian Gulf Crisis escalated following a United States of America (US) drone strike which killed Iranian Major General Qasem Soleimani of Iran’s Islamic Revolutionary Guard Corps in Iraq.Subsequently, on 5th January 2020, Iraqi lawmakers voted in favour of a draft law that ordered the government to request US troops to withdraw from Iraq.[1]  Iran referred to the strike as a ‘criminal act of attack’[2] and the Supreme Leader warned that ‘retaliation is waiting’.[3]  Such retaliation took place on 8th January 2020, when Iran launched ballistic missiles at two air bases containing American personnel in Iraq. 

In terms of legality, Article 2(4) of the Charter of the United Nations (UN Charter)[4] prohibits member states from the use of force, although the one exception to this rule is the inherent right to self defence, which unsurprisingly formed the basis of the Trump administration’s shifting justifications.  Under international law, any use of force in self defence must be immediately reported to the United Nations Security Council (Security Council),[5] while also being a necessary and proportionate[6] response to an armed attack.[7] 

Promptly, US officials sought to justify the strike with claims that ‘Soleimani was plotting imminent and sinister attacks on American diplomats and military personnel’.[8]  The repeated use of the word ‘imminent’ in similar statements confirmed that the US intended to rely on the contentious notion of anticipatory self defence as the primary legal basis for the strike, for which an ‘imminent attack’ is considered a requirement.[9]  However, the letter sent from the US to the Security Council focussed on self defence in response to an ‘escalating series of armed attacks’ instead,[10] employing the accumulation of events doctrine.

Both an imminent threat and an accumulation of attacks can, under international law, be equated to a sufficient ‘armed attack’ to justify self defence.  Although, as will be discussed, the lack of evidence of any imminent threat and the absence of any cumulatively substantial Iranian attacks against the US renders the latter’s victim claim unconvincing.  Furthermore, even if the US were able to convince the International Court of Justice (ICJ) that it was the victim of an armed attack, it is unlikely that the drone strike would be considered a necessary and proportional response.  Thus, this article will illustrate why the Trump administration’s justifications fail to uphold a convincing argument for what, under international law, certainly appears to be an illegal use of force.

The failure of the claim that the US acted in anticipatory self defence

From the offset, the Trump administration relied on the possibility of Soleimani ‘planning, coordinating and synchronising’[11] an imminent attack on US personnel both ‘in Iraq and throughout the region’[12] to justify the use of force as anticipatory self defence. However, there is excessive debate amongst scholars as to whether such a right actually exists.

Restrictionists argue that Article 51 of the UN Charter (Article 51) precludes anticipatory self defence, as it would ‘replace a clear standard with a vague, self-serving one, and open a loophole large enough to empty the rule’.[13]  Furthermore, Murphy contests that the phrase ‘armed attack’ indicates a more significant incident than a mere ‘use of force’, but it can be difficult to predict the exact consequences of an anticipated attack.[14] Due to the inability to foresee whether the effects of an attack will be ‘most grave’,[15] restrictionists conclude that anticipatory self defence is not covered by Article 51. 

Conversely, expansionist arguments reference The Caroline case, advocating that anticipatory self defence is a component of customary international law.  While the attack in Caroline was ongoing, supporters maintain that it set the time scope for anticipatory self defence as ‘instant, overwhelming, leaving no choice of means and no moment for deliberation.’[16]  Building on this, the ICJ held in Nicaragua that the customary international law right to self defence survives the UN Charter, existing alongside Article 51.[17] 

The absent reference to anticipatory self defence in Article 51 is sufficient for many commentators to substantiate that it is also included as a matter of international customary law.[18] Following analysis of the Six Day War[19] and the Israeli attack on the Osirak nuclear installation,[20] state practice appears to support this. Thus, while the strength of expansionist arguments confirms that there is possibility for the US to rely upon this measure as their legal justification for the drone strike, they would still have to prove that it was their ‘last window of opportunity’ to defend themselves against an ‘irreversible emergency’.[21] 

However, on 9th January 2020, the US Secretary of State, Mike Pompeo, told Fox News: ‘…we don’t know precisely when and we don’t know precisely where’[22] any so-called imminent attack was due to take place.  Moreover, contrary to an earlier statement given by the current US President Donald Trump, who claimed that the attack would have targeted embassies,[23] the US Secretary of Defence, Mark Esper, ‘didn’t see’ any evidence of such targets.[24] Concurrently, since the drone strike occurred, no evidence has materialised in support of the claim that Iran posed any imminent threat to the US.  Seeing as the burden of proof to illustrate the existence of an armed attack rests on the victim state,[25] the failure of the US to do so suggests that the justification of anticipatory self defence is weak and largely unfounded.

The inapplicability of the accumulation of  events doctrine to the US argument

In any case, it would appear that the Trump administration quickly accepted the failure of their anticipatory self defence argument, as the State’s letter to the Security Council did not mention any ‘imminent attack’; commentators have consequently argued that the letter ‘directly contradicts’ the prior justifications offered.[26] Instead, the justifications to the Security Council focussed on an ‘escalating series of armed attacks’,[27] as the US redirected its argument to the accumulation of events doctrine.  This theory allows for a series of less severe events to collectively amount to an “armed attack”. 

Nevertheless, only one of the armed attacks referred to in the series of incidents contained in the letter was actually performed by Iran, and this was their retaliatory action to the drone strike itself; the other attacks discussed were carried out by Iranian supported groups.  International law stipulates that only attacks attributable to Iran would be valid to enable the US to act in self defence against Iran[28] and thus, an accumulation of events composed of primarily rebel-conducted attacks cannot amount to an armed attack.  Notwithstanding this, it is arguable that attacks by non-state actors could be equated to an armed attack if Iran had effective control over them,[29] but the letter does not offer any indication that Iran instructed or controlled any of the identified militia actions.[30] 

Given the ICJ’s maintained, restrictive and orthodox position on self defence whereby Article 51 recognises the right ‘in the case of armed attack by one state against another State’,[31] its likelihood to allow these events to constitute an armed attack is minimal.  Furthermore, the facts at hand share similarities to the Oil Platforms case.[32] Here, the US relied on a ‘series’ of Iranian armed attacks to justify their claim of self defence[33] but the court held that even when taken cumulatively, the events did not constitute an armed attack[34] that qualified as a most grave use of force.  Seeing as the only attack that is attributable to Iraq occurred after the drone strike and, as discussed, it is the responsibility of the US to demonstrate its victimhood,[35] the application of the accumulation of events doctrine does not reveal any greater evidence of a substantial armed attack requiring defence.

Failure of the US to meet the requirements for the principles of necessity and proportionality

Despite the improbability of the ICJ recognising the claim that the US was the victim of an armed attack significant enough to trigger Article 51, it is nevertheless worth addressing the necessity and proportionality principles.  This universally accepted aspect of customary international law[36] asserts that any form of self defence must only engage measures that are ‘proportional to the armed attack and necessary to respond to it’.[37] In order for the drone strike to have been necessary, the US would need to prove that Soleimani’s death was the only way to prevent an imminent attack.[38]  However, Bateman contends that Soleimani was a decision-maker as opposed to an operational asset and thus, ‘killing him would neither be necessary nor sufficient to disrupt the operational progression of an imminent plot’.[39] 

Additionally, the strike took place in Iraq, a third party state, without Iraqi government consent, constituting a direct violation of Article 2(4) of the UN Charter. Milanovic elaborates that from an expansionist perspective, analysis of this should be analogous to self defence against non-state actors, such that the use of force in the third party state where the attacker is located could be justified by the necessity of stopping an attack in a situation where the state in question is unwilling and unable.[40]  Regardless, the US has neglected to prove that it was necessary to strike Soleimani at the exact time and location that it did, and that they were unable to ask the Iraqi government for permission. In fact, the letter submitted by the US to the Security Council effectively compounds this, admitting that the US was acting in response to past incidents with the intention to deter future ones. 

Furthermore, on January 5th 2020, the Prime Minister of Iraq revealed that Soleimani was in Iraq with the intention to de-escalate tensions with the US and requested that the Iraqi government act as mediator.[41] Thus, the necessity of the drone strike is brought even further into question. 

As a matter of proportionality, it has been argued that the killing of Soleimani resulted in the loss of far fewer lives due to the subsequent prevention of potential Iranian attacks,[42] but this is ultimately weakened by the aforementioned point that his death would not be sufficient to avert an imminent plot. In addition, this would depend on whether there has actually been an imminent attack, but the justifications of the Trump administration shifted from this possibility to the series of armed attacks, none of which would be likely to warrant the triggering of Article 51. Finally, although the US did notify the Security Council of their use of force in self defence, satisfying the third of the three conditions for the act to be lawful, this did not happen ‘immediately’, as required.[43] 

The differing justifications offered by the Trump administration during this delay brings the necessity of the strike under further scrutiny. The reasons for the need to act in self defence delivered in official statements evolved immensely in just a week, suggesting that perhaps there was no certain reason for the strike to begin with and therefore, no true necessity.


In summary, analysis of the US justifications for the drone strike reveals an unequivocally illegal use of force. In order for the strike to be considered lawful, it had to be: (1) immediately reported to the Security Council, (2) in response to an armed attack, and (3) a necessary and proportional response. Not only was there a five day gap between the incident and the notification of the Security Council, but the legal justifications provided by the US during this time were varied and contradictory. 

US officials initially insisted that they were facing an ‘imminent threat’ from Iran,[44] but were unable to provide evidence of this and thus, the anticipatory self defence argument is greatly debilitated. Following this, the US turned to an accumulation of Iranian events to evidence an armed attack. Again, this argument lacks strength on the basis that only one of the attacks listed was attributable to Iran and this occurred after the strike; the others were carried out by non-state actors and the US made no effort to demonstrate that Iran had effective control over their actions. As a result, there is no credible evidence of an armed attack. Finally, it was not proven necessary nor proportionate for the US to carry out this drone strike in Iraq without the State’s consent; killing Soleimani would have been exceedingly unlikely to prevent any imminent attacks as he was not considered as an operational asset. 

Therefore, the attempts by the Trump administration to fulfil the international law requirements for self defence have been exceptionally weak and thus, this article concludes that the US violated Article 2(4) of the UN Charter and the drone strike was an illegal use of force.

[1] ‘170 Iraqi MPs Sign Draft Law to Expel U.S. Troops from Iraq’ (Al-Manar News, 5 January 2020) <http://english.almanar.com.lb/905643> accessed 1 April 2020.

[2] Lyse Doucet, ‘Qasem Soleimani: US kills top Iranian general in Baghdad air strike’ (BBC News, 3 January 2020) <https://www.bbc.co.uk/news/world-middle-east-50979463> accessed 1 April 2020.

[3] ‘Soleimani killing: Iran abandons nuclear deal limits’ (Al Jazeera News, 5 January 2020) <https://www.aljazeera.com/news/2020/01/soleimani-killing-iran-abandons-nuclear-deal-limits-200105185905943.html> accessed 1 April 2020.

[4] United Nations, Charter of the United Nations (24 October 1945) 1 UNTS XVI, art 2, para 4.

[5] United Nations, Charter of the United Nations  (24 October 1945) 1 UNTS XVI, art 51.

[6] The Caroline v United States, 11 US (7 Cranch) 496 (1813).

[7] (n 5).

[8] Ryan Pickrell, ‘The Trump administration is struggling to explain why the US killed top Iranian general Soleimani — here’s all the shifting explanations’ (Business Insider, 13 January 2020) <https://www.businessinsider.com/trump-administrations-shifting-explanations-for-soleimani-killing-2020-1?r=US&IR=T> accessed 26 March 2020.

[9] Chatham House, Principles of International Law on the Use of Force by States in Self-Defence (ILP WP 05/01, 1 October 2005).

[10] Letter from the Permanent Representative of the United States of America to the President of the United Nations Security Council (8 January 2020).

[11] Jeff Schogol, ‘Milley: We’ll provide evidence of an imminent Iranian attack one day, somehow’ (Task and Purpose, 7 January 2020) <https://taskandpurpose.com/news/milley-evidence-imminent-iran-attack> accessed 27 March 2020.

[12] Ryan Pickrell, ‘Pentagon says it killed top Iranian general Qassem Soleimani on Trump’s order’ (Business Insider, 3 January 2020) <https://www.businessinsider.com/dod-says-us-military-killed-top-iranian-general-qasem-soleimani-2020-1?r=US&IR=T> accessed 26 March 2020.

[13] Louis Henkin, ‘International Law: Politics, Values and Functions. General Course on Public International Law (Volume 216)’ in Collected Courses of the Hague Academy of International Law (1989) <http://dx.doi.org/10.1163/1875-8096_pplrdc_A9780792310488_01> accessed 1 April 2020, 156.

[14] Sean D. Murphy, ‘The Doctrine of Preemptive Self-Defense’ (2005) 50 Vill L Rev 699 <https://digitalcommons.law.villanova.edu/vlr/vol50/iss3/9> accessed 1 April 2020, 709-10.

[15] Case Concerning Military and Paramilitary Activities In and Against Nicaragua (Nicaragua v United States of America)(Merits, ICJ Rep 14,1986) 101, para 191.

[16] The Caroline v United States, 11 US 7 Cranch 496 (1813); Letter from Secretary of State Daniel Webster to Lord Ashburton (6 August 1842).

[17] Case Concerning Military and Paramilitary Activities In and Against Nicaragua (Nicaragua v United States of America)(Merits, ICJ Rep 14,1986) 101, para 190.

[18]  Agbada S. Agbada, ‘Is the Killing of Qasem Soleimani by the United States of America Legal under International Law?’ (7 January 2020) <https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3515524> accessed 31 March 2020, 3-4.

[19] Richard N. Haass, ‘The Six Day War at 50’ (Project Syndicate, 23 May 2017) <https://www.project-syndicate.org/commentary/six-day-war-at-50-by-richard-n-haass-2017-05-by-richard-n–haass-2017-05?barrier=accesspaylog> accessed 1 April 2020.

[20] Sebastian Roblin, ‘How Israel and Iran Teamed up to Crush Iraq’s Nuclear Bomb Program’ (The National Interest, 4 August 2019) <https://nationalinterest.org/blog/buzz/how-israel-and-iran-teamed-crush-iraqs-nuclear-bomb-program-71051> accessed 1 April 2020.

[21] Chatham House, Principles of International Law on the Use of Force by States in Self-Defence (ILP WP 05/01, 1 October 2005).

[22] David Welna, “Imminent’ Threat – Trump Justification Of Attack on Iranian General – Is Undefined’ (NPR, 10 January 2020) <https://www.npr.org/2020/01/10/795438264/imminent-threat-trump-justification-of-attack-on-iranian-general-is-undefined?t=1586087126833> accessed 31 March 2020.

[23] Morgan Chalfant, ‘Trump Says Soleimani was plotting attacks on four US embassies’ (The Hill, 10 January 2020) <https://thehill.com/homenews/administration/477740-trump-says-four-embassies-targeted-before-soleimani-strike> accessed 1 April 2020.

[24] Peter Baker and Thomas Gibbons-Neff, ‘Esper Says He Saw No Evidence Iran Targeted 4 Embassies, as Story Shifts Again’ (New York Times, 12 January 2020) <https://www.nytimes.com/2020/01/12/us/politics/esper-iran-trump-embassies.html> accessed 1 April 2020.

[25] Case Concerning Oil Platforms (Islamic Republic of Iran v United States of America)(ICJ Rep 161, 2006) para 57.

[26] Eliot Engel, ‘Engel Statement On The White House’s Latest Justification For Soleimani Killing’ (US Representative Eliot Engel, 14 February 2020) <https://engel.house.gov/latest-news/engel-statement-on-the-white-houses-latest-justification-for-soleimani-killing/> accessed 1 April 2020.

[27] Letter from the Permanent Representative of the United States of America to the President of the United Nations Security Council (8 January 2020).

[28] Case Concerning Military and Paramilitary Activities In and Against Nicaragua (Nicaragua v United States of America)(Merits, ICJ Rep 14,1986).

[29] Application of the Convention on the Prevention and Punishment of the Crime of Genocide (Bosnia and Herzegovina v Serbia and Montenegro) (ICJ Rep (Merits) Judgment, 2007) para 397.

[30] Adil Ahmad Haque, ‘U.S. Legal Defense of the Soleimani Strike at the United Nations: A Critical Assessment’ (Just Security, 10 January 2020) <https://www.justsecurity.org/68008/u-s-legal-defense-of-the-soleimani-strike-at-the-united-nations-a-critical-assessment/> accessed 3 March 2020.

[31] Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory Advisory Opinion (ICJ Reports, 2004) para 136.

[32] Case Concerning Oil Platforms (Islamic Republic of Iran v United States of America)(ICJ Rep 161, 2006)

[33] ibid, para 62.

[34] (n 32) para 63.

[35] (n 25).

[36] Legality of the Threat or Use of Nuclear Weapons (Advisory Opinion, ICJ Rep 226, 8 July 1996) para 41.

[37] (n 28), para 176.

[38] Agnes Callamard, ‘The Targeted Killing of General Soleimani: Its Lawfulness and Why It Matters’ (Just Security, 8 January 2020) <https://www.justsecurity.org/67949/the-targeted-killing-of-general-soleimani-its-lawfulness-and-why-it-matters/> accessed 24 March 2020.

[39] Josh Dawsey, John Hudson, Dan Lamothe and Missy Ryan, ‘How Trump decided to kill a top Iranian general’ (The Washington Post, 4 January 2020) <https://www.washingtonpost.com/national-security/how-trump-decided-to-kill-a-top-iranian-general/2020/01/03/77ce3cc4-2e62-11ea-bcd4-24597950008f_story.html> accessed 2 April 2020.

[40] Marko Milanovic, ‘The Soleimani Strike and Self-Defence Against an Imminent Armed Attack’ (EJIL: Talk!, 7 January 2020) <https://www.ejiltalk.org/the-soleimani-strike-and-self-defence-against-an-imminent-armed-attack/> accessed 2 April 2020.

[41] (n 38).

[42] Alan M. Dershowitz, ‘Easy Call: The Strike on Soleimani Was Lawful’ (The Wall Street Journal, 5 January 2020) <https://www.wsj.com/articles/easy-call-the-strike-on-soleimani-was-lawful-11578261997> accessed 2 April 2020.

[43] (n 5).

[44] Mark Hosenball, ‘Trump says Soleimani plotted ‘imminent’ attacks, but critics question just how soon’ (Reuters everythingNews, 3 January 2020) <https://uk.reuters.com/article/us-iraq-security-blast-intelligence/trump-says-soleimani-plotted-imminent-attacks-but-critics-question-just-how-soon-idUSKBN1Z228N> accessed 3 April 2020.

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