The first chapter introduces the research puzzle of the monograph. The book investigates whether the contemporary debate on the meaning, objectives and scope of European strategic autonomy (ESA) and its compatibility with the roles of the United States of America and North Atlantic Treaty Organization (NATO) in the European security architecture presents a strategic challenge or a window of opportunity for small states to advance their national security interests in the European Union (EU)? Next, it establishes the theoretical framework, arguing that small states’ capabilities to pursue their national interests in international institutions mostly depend on the external factors, empowering or constraining the implementation of their foreign policy objectives regarding European strategic autonomy. Finally, the chapter presents the monograph’s scope, structure, and its case studies.