This article provides a comprehensive examination of the legal responsibility of states concerning international terrorism and the dimensions of counter-terrorism diplomacy. Recognizing the growing threat of terrorism to international peace and security, the research analyzes the international legal framework, including United Nations conventions, Security Council resolutions, and the principles of international law governing states' obligations. States' general obligations to prevent terrorist acts and refrain from supporting terrorist groups, as well as their specific obligations regarding judicial cooperation, extradition of offenders, and reporting to international bodies, are scrutinized. The paper also addresses existing legal mechanisms for countering terrorism, including the role of the UN Counter-Terrorism Committee, cooperation with Interpol, and the functioning of international and regional courts, such as the International Criminal Court. Major challenges, such as the lack of a unified international definition of "terrorism," the difficulty of proving state responsibility for supporting terrorism, and conflicts of jurisdiction between national and international courts, are examined in detail. Case studies, including the Nicaragua v. United States case at the International Court of Justice and an examination of aspects of Iran's performance in implementing international obligations, further illuminate the practical dimensions of these challenges. Finally, by offering recommendations to strengthen the legal framework – including formulating a unified definition of terrorism, establishing a more effective monitoring system, expanding the International Criminal Court's jurisdiction to terrorist crimes, and facilitating the extradition of criminals – this research emphasizes the necessity of balancing state sovereignty with collective counter-terrorism obligations. The article's main conclusion is that a multilateral approach, based on the rule of law and full respect for human rights, is indispensable for an effective and sustainable response to international terrorism