Jus ad bellum and jus in bello. One describes the conditions under which a state is permitted to resort to armed force, the other the way in which that warfare is conducted. Even if it is accaepted that a country was right to respond militarily to terror attack (rather than, for instance, negotiating with the terrorists to recover hostages) there are rules about how that response can happen.
For instance, hospitals and medical centres are to be protected. Attacking a hospital in such a way that everyone in the ICU dies would be a war crime.
Proportionality is to be observed. If, say, 1200 people, both soldiers and civilians, are killed it in a terror attack, killing ten times that number of civilians in response would be disproportionate (the country involved claiming that all members of that group, from newborn babies upwards, are ‘terrorists’ and thus legitimate targets does not make it so) and so a war crime.
Using a weapon like white phosphorus as an incendiary weapon directly against humans in a civilian setting would be a war crime.
Jus in bello is at least as important as jus ad bellum, and to ignore questions about how a war is being conducted, on the basis that a belligerent party ‘has the right to self defence’ is an attack on international humanitarian law and the supposed ‘rules-based order’.
Any government that ignores jus in bello because of their alliance with one of the belligerent parties has lost the right to pontificate about human rights to the rest of the world. Were such a country to, say, condemn the treatment of the Rohingya by Myanmar or the Uighurs by China, the retort could rightly be made that that country’s opposition to such ethnic cleansing is hypocritical in the light of its support for its ally’s ethnic cleansing.
Jon Pinter
