The Second World War was a multidimensional, complex conflict covering several large-scale and narrow armed conflicts which intertwined, interacted and influenced one another. Some of them had already been existent, starting before 1939; some were interstate conflicts that had not previously moved into the phase of armed confrontation, some did not emerge or become the pivot for the system of international relations until September 1939. In connection with this intertwining of conflicts it would appear necessary to clarify the question of the causes of the Second World War. In order to put forward a scientific hypothesis about the causal relationship between the structural specifi city of the system and the conditions for the start of the war, we should discuss, first of all, whether the system had a functional core (ensuring its stability) and what was its state in on the eve of the war. Another set of problems is related to the political poles (centers where power concentrated and the military-political decisions were made) in the system, as well as to the process of polarization (increasing confrontation of interests) of these centers. The proposed concept of the origin of the Second World War and the conditions for its outbreak is based on the thesis that the systemic state that emerged in the period between the two world wars was not multipolar. Polarization (as a process of a sharp divergence of interests within a group of the major powers) led to the emergence of bipolarity as a new macrostructural principle of system organization. At some stage the state of affairs was complemented by the change of the leading power at global level. The state of the system of international relations in 1939 acquired a fundamentally new macrostructural quality — incomplete (or cluster) bipolarity. This feature arose because the process of forming confl icting military-political alliances was far from being completed. Thus, the central complex of theoretical problems posed in the article will involve an assessment of the role that the accumulation of power and the struggle for leadership in this area played in shaping the conditions for the outbreak of World War II.