Practice Relating to Rule 140. The Principle of Reciprocity
Reprisals or retaliation under international law are also governed by certain specific principles … Reprisals could not involve acts which are malum in se
such as certain violations of human rights, certain breaches of the laws of war and rules in the nature of ius cogens
, that is to say obligations of an absolute character compliance with which is not dependent on corresponding compliance by others but is requisite in all circumstances unless under stress of literal vis major
… In other words … even where a wrongful act involved the use of a nuclear weapon the reprisal action cannot involve [the] use of a nuclear weapon without violating certain fundamental principles of humanitarian law. In this sense, prohibition of the use of a nuclear weapon in an armed conflict is an absolute one, compliance with which is not dependent on corresponding compliance by others but is a requisite in all circumstances.