This article focuses on the application of autonomous weapons (AWs) in defensive systems and, consequently, assesses the conditions of the legality of employing such weapons from the perspective of the right to self-defence. How far may humans exert control over AWs? Are there any legal constraints in using AWs for the purpose of self-defence? How does their use fit into the traditional criteria of self-defence? The article claims that there are no legal grounds to exclude AWs in advance from being employed to exercise the right to self-defence. In general, the legality of their use depends on how they were pre-programmed by humans and whether they were activated under proper circumstances. The article is divided into three parts. The first discusses how human control over AWs affects the legality of their use. Secondly, the article analyses the criteria of necessity and proportionality during the exercise of the right to self-defence in the context of the employment of AWs. Finally, the use of AWs for anticipatory, pre-emptive or preventive self-defence is investigated.

Recent developments in legal scholarship evidence that the orthodoxy on the law on the use of force has dramatically switched from a restrictivist to an expansionist perspective. This article seeks to analyse this recent shift, especially with respect to the law of self-defence, from an expansionist point of view. Its purpose is to examine the argumentative landscape which currently exists on the expansionists’ side about that law. It observes that such argumentative landscape has significantly changed, as expansionists tend to pay less attention to the traditional arguments based on state practice and increasingly rely on policy considerations in order to strengthen and to go deeper in their wide conception of the law of self-defence. It calls into question such increasing recourse to policy oriented arguments and argues that those arguments cannot justify alone any evolution of the law of self-defence, while emphasizing that state practice remains central in that respect and explaining the different ways through which this practice may play such a role.

While the overwhelming majority of states do not practice anticipatory self-defence, believing that it would create a dangerous precedent, it is ironic that many scholars in the area support the idea of it. After September 11, the legality of anticipatory self-defence is attracting increasing support. What makes the matter worst is the pronouncement of a preventive style of self-defence in the National Security Strategy of the US, which goes far beyond the traditional concept of ‘anticipatory’ self-defence. The old doctrinal debate has resurfaced with stronger vigour. The present article is an attempt to re-appraise the debate and test the legality of anticipatory self-defence. The article objectively interprets Article 51 of the Charter in light of canons of treaty interpretation and explores customary international law of the time. It re-examines the Caroline formula and affirms that it had not been established as customary international law before September 11, owing to the lack of widespread and consistent state practice and opinio juris. The article concludes that although state practice after September 11 tends to condone the use of force against imminent terrorists attacks, Article 51 of the UN Charter is still a good law that can cope with the normal inter-state use of force. Despite the flaws of the United Nations, the Charter-based system of world order can very well serve the international community even in the context of the 21st century world order.