National interests of any state are essentially national security interests. Basic (vital) interest of the Republic of Slovenia and any other contemporary state is ensuring the highest possible level of security of its citizens and institutions. The author finds out that the concept of national security interest has been changing in parallel with changing concept of security threats. Broadening an understanding of (threats to) security after the end ofcold war has brought to differentiation of national security interest, dividing it to a multitude of partial security interests that correspond to dimensions of security. In this respect, we face a problem of defining the national security interests. This is elaborated in the article by using the concept of defence dilemma and the analysis of the debate on relationship between security and liberty after the September terrorist attack on WTC and Pentagon. The author also explains that basic national security interest (of the Republic of Slovenia) is to be found in prevention of escalation of security threats in other states from the same region and in stopping their transnational spreading.