While an overwhelming majority of states have hitherto not exercised any form of anticipatory self-defense, believing that it may lay foundation for an ominous precedent, the question remains as to why some writers insist on promoting the concept. As of September 11, there have been increasing louder voices pronouncing its legality. Later, the introduction of the so-called preventive self-defense by the US as a part of its National Security Agenda, rested way beyond the traditional interpretation of the Anticipatory Self-Defense. It appears that the historic doctrinal debate has resurfaced with yet stronger vigor. The present article is an attempt to review the legality and re-appraise the debate on the notion of anticipatory self-defense.The article attempts to objectively interpret Article 51 of the Charter in light of the canons of treaty interpretation with the purpose of finding the customary international law of the time. It then reexamines the Caroline formula and affirms that in the absence of widespread and consistent state practice and opinio juris prior to September 11, it has failed to level up or form as a customary rule of international law. The article concludes that although state practice after September 11 tend to condone the use of force against imminent terrorists attacks, Article 51 of the UN Charter still stands as a valid and effective statute prevailing over the use force mechanism; and that regardless of the temporal flaws attributed to the International Community case history, a Charter-based world order may favorably serve even within the context of challenges ahead in 21st century. The article finally concludes that anticipatory self-defense has not been recognized as a standing norm in International law.