Cross-border airstrikes sit at the intersection of two different bodies of international law. The first asks whether a state was legally entitled to use force at all. The second governs how force must be used once an armed conflict exists.
Article 2(4) of the United Nations Charter requires states to refrain from the threat or use of force against the territorial integrity or political independence of another state. The two clearest Charter-based exceptions are force authorised by the UN Security Council under Chapter VII and individual or collective self-defence under Article 51 after an armed attack.
The hardest cases involve attacks by non-state armed groups based in another country. States that conduct cross-border strikes often argue that they are acting in self-defence against an ongoing or imminent threat. The state where the strike occurs may reject that argument and describe the action as a breach of sovereignty. International lawyers also debate when a threat is sufficiently immediate, whether the territorial state is unable or unwilling to address it, and whether the response is necessary and proportionate. There is no automatic rule that makes every cross-border counterterrorism strike lawful.
A separate body of rules, international humanitarian law, governs conduct during armed conflict. It requires distinction between military objectives and civilians, proportionality in attacks and precautions to reduce civilian harm. These rules apply independently of the argument over who was legally entitled to start using force.
That distinction matters. A state may claim that a strike was justified as self-defence, while questions can still arise about the target selected, the weapons used and civilian casualties. Conversely, compliance with the rules on targeting does not by itself prove that crossing the border was lawful.
For readers following Pakistan-Afghanistan or other cross-border conflicts, the safest way to understand legal claims is to separate three questions: Was there a lawful basis to use force? Was the response necessary and proportionate? And were civilians and civilian objects protected as required by the laws of war?