The relevance of the subject under study is conditioned upon the scientific originality and practical significance of the implementation of the defence function of the state in the context of drastic changes in the forms and methods of conducting combat operations in modern wars. At the general theoretical level, the structure of the mechanism for implementing the defence function of the state was analysed and methodological aspects of the correlation of its main elements were highlighted. The purpose of this study was to highlight the methodological characteristics of the concept, essence, and content of the mechanism for implementing the defence function of the state, to determine its place and role in protecting state sovereignty, territorial integrity, and other national interests during the full-scale aggression of the Russian Federation against Ukraine. The methodological toolkit of this study includes a set of research approaches, philosophical, general scientific, and concrete scientific methods that provided substantiated study results regarding the mechanism of implementing the defence function of the state. The theoretical design of this mechanism was analysed, as well as certain aspects of interaction and complementarity of its structural elements were considered. It was found that the model of this mechanism comprises legal (regulatory), institutional (organizational), and functional components. The regulatory element of the mechanism for implementing the defence function of the state covers the principles and norms that ensure the regulation of legal relations in the field of preparation for armed defence and protection in the event of an armed conflict (aggression). It was established that special subjects serve as the basis of the institutional (organizational) element of this mechanism, which, within the limits of their powers, carry out defence activities aimed at preparing for armed defence and protection in the event of an armed conflict (aggression). It was noted that defence activity is a functional component of the mechanism for implementing the defence function, which is implemented by authorized entities in appropriate forms, methods, and makes provision for preparation for armed defence in the event of armed aggression (conflict). The practical value of this study lies is that based on a methodological analysis of the legal literature and regulations of Ukraine on certain aspects of the implementation of the defence function of the state, proposals were formulated to create a realistic strategy for countering the aggressor state, aimed at improving the current legislation in the fields of national security and defence