This article deals with the problem of war and peace in the works of representatives of the social democratic thought of the 20th and 21st centuries. The focus is on the works of Marxists (K. Kautsky, A. Bebel, R. Luxemburg, F. Mehring, G.V. Plekhanov) and neo-Marxist I. Wallerstein, as well as their opponents: realists (C. von Clausewitz, R. Aron, and others) and supporter of liberal idealism W. Wilson, as well as on some studies of contemporary authors. Much attention is paid to defining the

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... ssence of wars, their causes, ways to achieve peace, and ideas about the ideal world order. Further, the evolution of social democratic theory on the issue of war and peace in the 20th century is traced. The views of Marxist theorists are compared with those of representatives of other areas of the science of international relations. It is concluded that socialist theory, which is based on the understanding of the essence of war laid down in the works of the founders of Marxism and has much in common with the realist school, has evolved towards liberalism in matters of peace and post-war world order. At the same time, it made a significant contribution to the theory of international relations and not only became the basis for neo-Marxism, but also enriched political realism and liberalism as well as modern science in general with some conclusions. Works of researchers of the 21st century develop and summarize the most important conclusions on war and peace made by representatives of Marxism and other schools of the theory of international relations, which indicates the aspiration of the world scientific community to solve this global problem through joint efforts.