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.