The research studies the formation of materialist “ethics of war” in Belarusian Soviet philosophy in the 1920s. It has been shown that despite the absence of special philosophical works on ethics during this period, the specificity of materialist ethics can be reconstructed on the basis of historical study of philosophy, in particular the Marxist interpretation of the philosophy of B. Spinoza. The specificity of the project of Marxist geneonomy as philosophical and sociological doctrine of family and marriage is analyzed. The geneonomy was developed in the 1920s by researchers from Minsk during the processes of transformation of family life and gender relations unfolding in the Soviet Union. These works combined socio-historical, philosophical and normative approaches for studying social phenomena, including war. The development of Marxist geneonomy is presented as one of the cases of a more general process of transferring foreign ideas and concepts in Belarusian social sciences and philosophy. The features of the geneonomic project and the role of the concepts of war and struggle, as well as their ethical assessment in the works of F. Müller-Lyer, whose ideas were adopted and transformed by Soviet researchers, are analyzed. The specificity of the materialist version of geneonomy as a scientific project is revealed. According the geneonomic works of the Belarusian philosopher S.Ya. Wolfson peculiarities of understanding war as a factor of social evolution throughout human history, analysis of the role of war in the dynamics of capitalist and Soviet societies and the development of family and marriage relations in them, the use of rhetorical means associated with the description of war and struggle for the analysis of scientific and philosophical activities were reconstructed. The main peculiarities of the Marxist-materialist version of the “ethics of war”, developed within the framework of the Belarusian Soviet philosophy and Marxist sociology in the 1920s, are shown. They are naturalistic and even sociologist vision of war; determinism in the acceptance of wars as an inevitable factor in the evolution of society; a dialectical assessment of the causes of war and its results; emphasis on the role of the collective good in the analysis of war and its consequences.