This paper examines the causes of the Russo-Japanese War of 1904–1905, the conflict that affected the historical fate of both Russia and Japan. The relevance of the research is that there are still contradictions between Russia and Japan, the conflict of interests related to the ownership of the Kuril Islands. The purpose of the paper is to study the causes of the Russo-Japanese War of 1904-1905 in order to empirically test some of the provisions of classical political realism with the help of its theoretical toolkit. In the author's opinion, such analysis allows one to focus on the most important things in order to answer why it was not possible to solve the conflicts between Russia and Japan and avoid the war. To achieve the goal, the author sets several research tasks, among which he emphasizes comprehension of the theoretical heritage of classical political realism in the context of the Russo-Japanese war, application of the provisions of this theoretical paradigm to the historical events preceding the war, study of the immediate causes of the conflict. In his article, the author uses both the works by classical political realists and a vast bibliography of the the Russian Japanese war of 1904-1905. The article examines the international situation in Asia, the relationship of countries in this region, their interests, on the basis of which one or another subject's behavior can be explained. In the study, the author leads to the conclusion that the war was made possible for several reasons: imperialism; force, as the main indicator, and the consequent power politics; the use of contradictions between countries for their own purposes; uncompromising adherence to national interests.