The alliances, throughout the history, have been and are a central and constant phenomenon in international policy. Reflecting on the important role of alliances in world politics, in the international relations litterature we find an impressive list of studies, articles and analysis in this area of research. However, standing out the fact that there is no a broad consensus on the concept of alliance, a further analysis of specific aspects of a political-military alliance is actual. In the given article, a number of definitions and considerations of this concept are presented. At the same time, the assessment of difficulties of defining the concept of military and political-military alliance allow us to highlight concepts such as national defense, collective defense, common defense and collective security, which actually determine the correlation between the concepts of national security and political-military alliance. Finally, we conclude the fact that in the current security environment, the political-military alliances are tasked to defend the common fundamental interests and values as well as national interests against any agression, maintaining the international security and stability in accordance with the UN Charter.