Cameron and The Caroline: Are UK drone strikes on ISIS “self-defense”?

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It’s no secret that the recent drone strikes in Syria are controversial, with legal scholars and various national governments taking widely divergent stances on the legality of such aerial strikes in “self-defense”.[1] Per the United Nations’ Charter, member states have the right to act in self-defense if an armed attack occurs against them.[2]

However, the United Kingdom had never seemed inclined to use drone strikes until Prime Minister David Cameron confirmed that the government authorized a drone strike in Syria on August 21 that killed two Britons who were fighting alongside the Islamic State, also known as “ISIS” and “ISIL”.[3] Now, some critics fear the British government is following the United States’ more liberal use of drone strikes, which has drawn extensive international scrutiny.[4]

Drone strikes’ legality depends on the interpretation of “self-defence” in Article 51 of Chapter 7 of the UN Charter.[5] The United Kingdom’s interpretation of Article 51 is that self-defense is valid when a threat is imminent and force is the only means to stop the threat.[6] A 19th-century doctrine known as the Caroline test is largely responsible for expanding the understanding of self-defense to include anticipatory self-defense (PDF) and for broadening the interpretation of Article 51.[7] The Caroline test originated with the so-called Caroline incident” in 1837 when British forces crossed into the United States without permission to intercept and set fire to a steamboat called the Caroline.[8] Canadian rebels fighting to overthrow British rule and stationed on British territory near Niagara Falls were using the ship for “revolutionary purposes” against Britain, with at least tacit support from the United States.[9] Through a series of letters between Alexander Baring, a Special Minister sent by Great Britain, and U.S. Secretary of State Daniel Webster, the two countries reached an understanding on the necessary criteria for anticipatory self-defense, criteria now recognized as the “Caroline test", and the matter was later dropped.[10]

Under the Caroline Test, a government must show “a necessity of self-defence, instant, overwhelming, leaving no choice of means, and no moment for deliberation.”[11] The test was reaffirmed after World War II by the Nuremberg Tribunal.[12] In 1940, despite a treaty and a strong understanding established the year before, the Third Reich invaded Denmark and Norway and later claimed self-defense. The tribunal rejected that argument, citing the Caroline test for anticipatory self-defense.[13] The tribunal concluded that the German government was deceitful and planned to use force while merely claiming peaceful intentions, which is inconsistent with the criteria in the Caroline test.[14]

In a recent speech to Britain’s House of Commons, Cameron asserted that the August 21 airstrike was an act of self-defense with no alternatives, and that it was a matter of national security: Cameron said the men killed by the strike “were planning to attack commemorative events in the U.K.”[15] Cameron told the chamber that he consulted with the U.K. Attorney General and was assured that the attack was lawful and had a basis in international law.[16]

Though Cameron justified the attack as anticipatory self-defense, using the Caroline rationale, because the threat was imminent and no other alternatives existed, the Prime Minister offered few details beyond that.[17] This caused many observers to question whether the threat was truly imminent and the drone strikes lawful.[18] Opponents are worried the United Kingdom will adopt the U.S. “strategy of a global war on terror.”[19] The U.S. takes a preventive self-defense stance, which is opposed by world powers including China, Germany, France, Australia, Japan, India, and the U.K.[20] In international law, there are three types of self-defense. First, there is a pure self-defense, as laid out explicitly by Article 51, where a country can defend itself from an actual armed attack.[21] Second, a country can act in anticipatory or pre-emptive self-defense as exhibited in the Caroline test, which countries have used to interpret and extend Article 51.[22] The final category, preventive self-defense, is the most controversial, with the United States as the primary advocate for its legality.[23] Preventive self-defense is different from preemptive self-defense in that preemption requires the threat to be imminent, while preventive does not.[24] If it turns out the threat to the United Kingdom was not imminent in this instance, Cameron could argue for preventive self-defense alongside the U.S., though that legal argument would presumably be weaker.[25]

Preventive self-defense has never been Britain’s stance,[26] nor had the U.K. directed a drone attack against a British citizen until last month.[27] Notwithstanding Britons’ skeptical and anxious reactions to the drone strikes, Defense Secretary Michael Fallon has said the United Kingdom would “not hesitate” to launch further secret drone strikes in Syria if necessary to thwart potential terror plots.[28]

[1] Nicholas Watt et al., David Cameron Faces Scrutiny Over Drone Strikes Against Britons in Syria, The Guardian (Sept. 8, 2015, 6:08 AM),

[2] United Nations Charter, Article 51, available at

[3] Watt et al, supra note 1.

[4] Laura Smith-Spark, Cameron Faces Questions Over UK Drone Strike in Syria, CNN (Sept. 8, 2015, 6:50 PM),

[5] Article 51.

[6] See Kevin Jon Heller, Why Preventive Self-Defense Violates the UN Charter, Opinio Juris (Mar. 7, 2012, 3:34 AM),

[7] See Frederic L. Kergis, Pre-emptive Action to Forestall Terrorism, Am. Soc’y Int’l L. (June 2002), available at (PDF).

[8] Louis-Phillipe Rouillard, The Caroline Case: Anticipatory Self-Defence in Contemporary International Law, 1 Miskolc J. Int’l L. 104, 104-20 (2004), available at

[9] Id.

[10] Id.

[11] Daniel Webster, Enclosure 1-Extract from note of April 24, 1841, reprinted in British-American Diplomacy: TheCaroline Case, at Enclosure 1 (Hunter Miller ed., 1934), available at

[12] Kergis, supra note 7.

[13] Id.

[14] Id.

[15] Watt et al, supra note 1.

[16] Id.

[17] Owen Bowcott et al., Right of Self-Defence Central to Legal Debate Over Syria Drone Strike, The Guardian (Sept. 7, 2015, 3:35 PM),

[18] Id.

[19] Id.

[20] Heller, supra note 6.

[21] Article 51, supra note 2.

[22] Webster, supra note 11.

[23] Heller, supra note 6.

[24] Id.

[25] Stephen Castle, Britain Won’t Hesitate on Drone Strike in Syria, N.Y. Times (Sept. 8, 2015),

[26] Heller, supra note 6.

[27] Cardiff Jihadist Drone Death Justified, Says Carlile, BBC News (Sept. 8, 2015),

[28] Id.

Posted by Amanda M. Hayes on Wed. September 23, 2015 9:41 PM
Categories: Insurgency, Islamic State, Law of War, Syria, Terrorism, United Kingdom, United Nations

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