WHEN a suicide bomber struck a mosque in Islamabad in February and militants stormed Bajaur days later, Pakistan confronted a question many states have faced before it: what does international law permit when those who kill your citizens plan, train and shelter across an international border? Kabul has condemned Pakistan’s strikes in Nangarhar, Paktika and beyond as violations of Afghan sovereignty. The law, however, is neither silent nor one-sided. It sets demanding conditions and a state that meets them stands on firm legal ground. The starting point is Article 2(4) of the UN Charter, which prohibits the use of force against any state’s territorial integrity. Article 51 gives the exception, preserving the “inherent right of individual or collective self-defence if an armed attack occurs”. The word “inherent” is deliberate. The Charter did not create the right; it recognized one that already existed in customary international law.
The harder question is whether that right extends to attacks by non-state actors operating from another state’s territory. In its 2004 Wall advisory opinion and in DRC v Uganda in 2005, the International Court of Justice took a restrictive view, tying self-defence to attacks attributable to a state. State practice, however, has moved decisively since 2001. Security Council Resolutions 1368 and 1373 affirmed the right of self-defence in the context of terrorist attacks and Resolution 1373 binds every state to deny safe haven to those who plan, finance or commit terrorist acts. Resolution 2249 of 2015 went further, calling on states to take all necessary measures against ISIL.
A victim state has two legal routes. The first is attribution. Article 3(g) of the Definition of Aggression, General Assembly Resolution 3314, treats the sending of armed bands or a state’s substantial involvement in such action, as aggression. The Friendly Relations Declaration of 1970 forbids states from acquiescing in organized activities on their territory directed at terrorist acts abroad and in Corfu Channel the ICJ held that no state may knowingly allow its territory to be used for acts contrary to the rights of others. Under Article 11 of the International Law Commission’s Articles on State Responsibility, conduct a state acknowledges and adopts as its own becomes attributable to it. Where a government shelters, tolerates and refuses to restrain a group whose fighters cross the border to kill, the distinction between host and sponsor begins to dissolve. The second route is the “unwilling or unable” standard. Where the territorial state cannot or will not suppress the threat, the victim state may act directly against the non-state actor, targeting the group rather than the host government.
The world offers ample precedent. The United States notified the Security Council under Article 51 on 7 October 2001 before striking Al-Qaeda in Afghanistan and in 2014 it invoked the unwilling or unable test explicitly against ISIL in Syria, joined by the United Kingdom, France, Australia and Türkiye. Kenya relied on self-defence against Al-Shabaab in Somalia in 2011. Türkiye has pursued the PKK into northern Iraq for decades on the same basis. When Israel struck Hezbollah in Lebanon in 2006, many states accepted its right to respond while criticizing the scale of the response. Colombia’s 2008 raid on FARC positions inside Ecuador, by contrast, drew censure from the Organization of American States, a reminder that the doctrine is not a blank cheque.
From this body of law and practice, the circumstances permitting cross-border force are clear. First, there must be an armed attack of sufficient gravity; the ICJ in Nicaragua distinguished the gravest forms of force from lesser frontier incidents, although a sustained campaign may be assessed cumulatively. Second, force must be necessary, a genuine last resort after peaceful avenues have failed, a principle traceable to the Caroline incident of 1837. Pakistan’s record here is substantial: repeated demarches, shared evidence, bilateral talks, third-party mediation and successive UN Monitoring Team reports documenting the TTP’s presence in Afghanistan. Third, the response must be proportionate to the threat it seeks to remove, not retaliatory, as the Court affirmed in its Nuclear Weapons opinion and in Oil Platforms. Fourth, the state must report its measures immediately to the Security Council, as Article 51 expressly requires.
Lawful resort to force does not suspend the law governing its conduct. The Geneva Conventions, which Pakistan ratified in 1951 and the customary rules of distinction, proportionality and precaution apply to every strike. In this domain, each allegation of civilian harm, substantiated or not, is weaponized in the information war. Rigorous targeting, transparent investigation and public accountability are therefore not concessions; they are strategic necessities.
Consistency matters too. Pakistan rightly rejected India’s strikes in 2019 and May 2025 because they rested on assertion rather than evidence and on no credible legal basis. Pakistan must hold itself to the standard it demands of others: evidence placed on record, prompt reporting to the Security Council and force aimed narrowly at terrorist infrastructure rather than at the Afghan people.
Sanctuary is not sovereignty. A state that allows its soil to become a launching pad for violence against a neighbour cannot invoke territorial integrity as a shield for that violence. Yet the right to strike back is only as strong as the legal case that accompanies it. In modern conflict, legitimacy is itself a weapon and Pakistan must wield it as carefully as any other.
—The writer is an international law expert and an internationally accredited arbitrator and mediator.
