The prohibition on the use of force constitutes a central pillar of contemporary international law. The purpose of the Charter of the United Nations is stated in Article 1: to maintain international peace and to suppress acts of aggression. Article 2(4) of the Charter establishes a general prohibition on the threat or use of force in international relations against the territorial integrity or political independence of any state. The only limited exception to this is provided in Article 51 of the UN Charter, which preserves the right to self-defence. Considered an ’inherent right’ for states, self-defence is the only justifiable rationale for the use of armed force without UN Security Council approval. However, the legality of anticipatory self-defence (i.e., pre-emptive military force) remains contentious.
Under Article 51 the precise scope of the self-defence exception has been the subject of considerable debate in judicial interpretation. The judgment of the International Court of Justice in Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America) (Merits) Judgment, ICJ Reports 1986, provides the most influential judicial interpretation of Article 51 of the UN Charter. Nicaragua alleged that the United States violated international law by supporting armed insurgents and conducting military operations against Nicaraguan territory. The United States argued that its actions were justified as collective self-defence on behalf of neighbouring states allegedly attacked by Nicaragua.
The Court rejected this proposition and held that only the most grave forms of force constitute armed attacks capable of giving rise to the right of self-defence under Article 51. Further, that providing financial or logistical support to rebel groups may constitute unlawful intervention but does not necessarily amount to an armed attack. Applying these principles, the Court concluded that the United States had not demonstrated the existence of an armed attack and therefore had violated international law.
The famous, and foundational, articulation of the law relating to anticipatory self-defence is found in the Webster-Ashburton Correspondence of 1842, (in the Caroline Case) comprising diplomatic exchanges between US Secretary of State Daniel Webster and British envoy Lord Ashburton. Webster laid down the test that a state claiming self-defence to justify an incursion into another sovereign’s territory must demonstrate ’a necessity of self-defence, instant, overwhelming, leaving no choice of means, and no moment for deliberation’.
A central tension exists between the Caroline doctrine and Article 51 of the UN Charter, which permits self-defence only ’if an armed attack occurs’. The Caroline test[3] applies precisely where Article 51 is not directly operative — i.e., where defensive action is taken before an armed attack has materialised. Restrictive interpreters argue that Article 51 supplanted the customary right of anticipatory self-defence; expansionists maintain that Article 51 preserved the pre-existing customary right, of which the Caroline formula[4] is the leading expression.
The International Court of Justice has never formally ruled on whether anticipatory self-defence is lawful. However, the ICJ has repeatedly and unequivocally endorsed the concepts of necessity and proportionality as elements of the customary international law governing self-defense, stating that the submission of self-defence to those conditions ’is a rule of customary international law’ — without expressly citing the Caroline test by name.
The reasons and evidence advanced by the U.S. administration do not appear to meet the legal threshold necessary to invoke the right of self-defence, either in the strict application of Article 51 per Nicaragua, or the ’imminent threat’ standard in the Caroline Case[6] to justify the military strikes against Iran. Secretary of State Marco Rubio admitted that the strikes were intended to ’preemptively go after them before they launched those attacks,’ while President Trump stated he ordered the strikes because it was his ’opinion’ that Iran was going to attack first. According to Caroline, the threat needs to be temporally imminent (within a few hours or a day) according to a very high standard of evidence, even alternatively in lieu of a more permissible understanding of the imminence threshold in modern day non-conventional warfare, states are still required to demonstrate the necessity of stopping a devastating threat under the pre-emptive self-defence doctrine. Thus, the claims of immediate or medium-term threats posed by Iran in terms of its ballistic missile capabilities or nuclear program not supported by the available evidence does not trigger the necessity requirement under modern day interpretation of pre-emptive self-defence.
In the absence of a valid claim to pre-emptive self-defence, having failed to demonstrate the requisite imminence, the necessity of military strikes loses their legal justification under international law. Consequently, rather than a lawful defensive measure, the US actions appear to amount to an unlawful use of force. The gravity of this classification cannot be overstated; the Nuremberg Tribunal described aggressive war as the ’supreme international crime…’
Maleeka Bokhari is a barrister at Red Lion Chambers and serves on the Board of Trustees of the International Bar Association Human Right Institute Trust (IBAHRI)
Sailesh Mehta is a barrister at Red Lion Chambers and founding member of the Bar Human Rights Committee and its first vice‑chairman
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