[1] Examples of state practice are given by Professor Antonio Cassese, former President of the International Criminal Tribunal for the Former Yugoslavia, in International Law, (Oxford, 2001) at 309-31. One particularly relevant example is the international reaction to an Israeli bombing attack on an Iraqi nuclear reactor: 'When the Israeli attack on the Iraqi nuclear reactor was discussed in the [Security Council], the USA was the only State which (implicitly) indicated that it shared the Israeli concept of self-defence. In addition, although it voted for the SC resolution condemning Israel (resolution 487/1991), it pointed out after the vote that its attitude was only motivated by other considerations, namely Israel's failure to exhaust peaceful means for the resolution of the dispute. All other members of the SC expressed their disagreement with the Israeli view, by unreservedly voting in favour of operative paragraph 1 of the resolution, whereby '[the SC] strongly condemns the military attack by Israel in clear violation of the Charter of the UN and the norms of international conduct.' Egypt and Mexico expressly refuted the doctrine of anticipatory self-defence. It is apparent from the statements of these States that they were deeply concerned that the interpretation they opposed might lead to abuse. In contrast, Britain, while condemning 'without equivocation' the Israeli attack as 'a grave breach of international law', noted that the attack was not an act of self-defence. Nor [could] it be justified as a forcible measure of selfprotection.''(p310).
[2] Cassese concludes that, '[i]f one undertakes a perusal of State practice in the light of Article 31 of the Vienna Convention on the Law of Treaties, it becomes apparent that such practice does not evince agreement among States regarding the interpretation or the application of Article 51 with regard to anticipatory self-defence.' (International Law (Oxford, 2001) at p309).
Oppenheim states that: 'while anticipatory action in self-defence is normally unlawful, it is not necessarily unlawful in all circumstances, the matter depending on the facts of the situation including in particular the seriousness of the threat and the degree to which pre-emptive action is really necessary and is the only way of avoiding that serious threat; the requirements of necessity and proportionality are probably even more pressing in relation to anticipatory self-defence than they are in other circumstances.' (R Jennings QC and A Watts QC (eds), Oppenheim's International Law: Ninth Edition 1991 pp41-42)
Detter states that, 'it must be emphasised that anticipatory force falls under the prohibition of force in Article 2(4) of the Charter entailing a presumption that it is illegal. A mere threat of attack thus does not warrant military action...' (The Law of War, Second Edition, (Cambridge, 2000), p86).
Cassese also considers that, '[i]n the case of anticipatory self-defence, it is more judicious to consider such action as legally prohibited while admittedly knowing that there may be cases where breaches of the prohibition may be justified on moral and political grounds...' (International Law, (Oxford, 2001), p311).