What are the international‑law standards for the use of force against non‑state actors operating from another state's territory?

1. The Charter baseline and the narrow exceptions

The core rule is that states “shall refrain” from the threat or use of force against another state’s territorial integrity or political independence, embodied in Article 2 of the UN Charter, and only two clear exceptions exist inside the Charter: force in self‑defence under Article 51 and Security Council authorization ^{[2] [1]}. Legal commentators and official practice therefore begin any analysis of cross‑border strikes against non‑state actors by asking whether one of those exceptions applies, and that framing constrains permissive readings of unilateral uses of force ^{[1] [7]}.

2. Attribution: when a non‑state actor becomes “state” conduct

A dominant line of authority requires that an armed attack by non‑state actors be attributable to a state before Article 51 can be invoked—classically using the International Court of Justice’s “effective control” or related tests—so that self‑defence remains primarily an inter‑state concept unless attribution can be shown ^{[4] [5]}. Tribunals and scholars have debated whether “overall control” or a sliding scale of control better fits modern practice, and the ICTY and other bodies have suggested nuance in applying control thresholds depending on context ^[4].

3. Necessity, proportionality and the evidentiary burden

Even where an armed attack is found or attributed, any responsive force must meet the fundamental jus ad bellum restraints of necessity and proportionality: force must be necessary to repel the threat and proportionate to the attack or imminent danger posed ^{[5] [8]}. Many observers stress that the victim state bears a high burden to demonstrate imminence, necessity, and degree of proportionality, and some scholarship argues for an even higher burden when operations intrude on a third state’s sovereignty ^{[9] [8]}.

4. The “unwilling or unable” doctrine and state practice

A competing, more permissive approach embraced by several states and commentators posits that a victim state may use force in another state’s territory when the host is “unwilling or unable” to prevent attacks emanating from its soil; proponents treat this as an emerging customary rule, while critics warn it risks undermining the Charter if applied too broadly ^{[9] [3]}. Empirical studies and policy papers show inconsistent state practice on this doctrine, meaning its legal standing remains contested and fact‑sensitive ^{[3] [10]}.

5. Due diligence, sovereignty and non‑use norms

Independent of the self‑defence line, international law imposes due diligence obligations on host states to prevent their territory from being used to harm others, and failure to exercise such diligence is central to arguments justifying limited extraterritorial force or Security Council action ^{[4] [11]}. But courts like the ICJ have historically been cautious about creating broad exceptions and some scholars read existing jurisprudence as still preventing unilateral cross‑border force absent clear attribution or UN authorization ^{[6] [10]}.

6. The practical and legal landscape: unsettled, politicized, and precedent‑driven

In short, the legal standards are a patchwork: Article 2 establishes a firm prohibition, Article 51 and UNSC mandates are narrow exceptions, attribution tests (effective/overall control) and necessity/proportionality are decisive, and the “unwilling or unable” formulation reflects state practice without settled legal weight—meaning real‑world decisions often turn on political judgments, evidentiary showings, and whether states seek Security Council cover or risk legal dispute ^{[1] [4] [3] [5]}. Because authoritative adjudication (e.g., the ICJ) has not definitively accepted a standalone right to use force against non‑state actors on foreign soil, the law will continue evolving through state practice, scholarship, and any future judicial rulings ^{[6] [8]}.