The acts of terrorism committed by non-state actors on September 11, 2001 were promptly responded by the use of force against Afghanistan, thus, commencing the ‘war on terror.’ This thesis sets out the essential components of the international legal framework i.e. jus ad bellum and jus in bello against which the September 11 attacks and the lawfulness of measures taken in response, thereto, fall to be assessed. The main emphasis remains on relevant laws in relation to the responsibility for 9/11 terrorist attacks, lawful constraints on the use of force in self-defense, limitations on conduct of hostilities under the humanitarian principles and safeguards provided by law-enforcement paradigm to protect basic human rights. This study is, therefore, to highlight the existence of international legal regime capable of addressing the events such as September 11 and governing the responses to that. It enquires the compatibility of the ‘war on terror’ with the legal framework provided by international law especially the Law of Armed Conflict in order to find out the legitimacy of use of force in preemptive self-defense, conduct of military operations to prevent transnational terrorism, targeted killings, collateral damage, detention operations and prosecution procedures. It also refers to the implications for states responsible for noncompliance to the law and subsequently for international rule of law. In this regard, the study explores that interpretation of the law is significant that regulates the state-based rationale behind compliance or noncompliance to the law. If there is a state-will to focus on humanitarian protection of individuals then rationale is more related to the applicability of the law, and if a state uses its influence and powerful status to achieve political-military objectives then rationale is less concerned about the rule of law. This proposition constitutes the threshold of applicability of the law in the ‘war on terror'' and eventually leads to the conclusion that unilateral use of force to counter terrorism is not a viable solution rather it obscures international law as a legal framework in such military operations. This thesis, thus, maintains that the existing international norms and standards can only establish the rule of law, if states interpret the law in its simplest form, apply it regardless of their vested interests and pay due respect to international legal regime.