Jean-Marie Henckaerts Responds to Ken Anderson on ICRC Customary International Humanitarian Law Study

An accurate snapshot of customary IHL does not merely require a description of what parties “actually do in actual wars”. This is so because customary international law is formed by a general practice accepted as law. The latter element is often referred to as “opinio juris”. The requirement of a general practice, combined with evidence that this practice conforms to a legal obligation has two important implications:

1. To the extent that field practice is not official and does not represent the legal conviction of the State concerned, it does not count. This implies, in particular, that if actual practice is generally seen as a violation of existing rules, this practice is not of a nature to modify existing rules. That is why notwithstanding numerous reports of, for example, attacks against civilians, pillage and sexual violence these acts are still prohibited under customary international law. The conclusion that these acts are considered to be violations of existing rules can be derived i.a. from a number of verbal acts, such as legislation, case-law and official statements. These verbal acts are therefore important and have to be considered to get the full and correct picture of customary international law.

2. The opposite is also true. Although States may in practice abstain from engaging in certain behaviour, through their verbal acts, qualifying their abstention as mere policy not based on a legal obligation to abstain, they can reserve their right to engage in those acts in the future. That is why attacks against works and installations containing dangerous forces – although seldom or never resorted to in the last twenty years – would still be considered lawful by States in case they constitute military objectives and sufficient precautions are taken. If mere battlefield behaviour were examined such targets would have to be considered off-limit. The same is true for nuclear weapons: on the basis of a mere consideration of battlefield practice, nuclear weapons would have to be considered unlawful – a position clearly not shared by the nuclear powers. This shows that verbal acts have an important impact on how battlefield practice has to be looked at.