The literature about Brazilian foreign policy before and throughout the Second World War and about the inner motivations that caused Brazilian involvement in the struggle, is lengthy and has renowned quality. The literature concerning the strategic aspects of the War, the Great Powers policies and interests, and their interactions, is of equal or indeed more value. The main goal of this work is to show how much the latter aspect decisively influenced the former. How the international system, in process and completely plunged in a total and global war, acted on the international actors, and how those constraints were the primary cause for the understanding of the involvement and behavior of the peripherical actors of the international system during the war. It s in this backdrop that Brazilian case is analyzed. Throughout the three chapters, the national foreign policy is described and explained, from the period of commercial diplomacy, immediately before the war s outset in Europe, through the gradual Brazilian engagement in the conflict between 1939-42, that realized real gains for the country, to the formal declaration of war to the European Axis, in August 1942. The main reason for this is the existent constraints in the extremely conflicted international system, encompassing all the planet.