The Second Osean War, known also as the Great Osean War or the Osean-Belkan War, started on 11 October 1937 with a full-scale invasion of the Osean Federation mainland by the combined arm forces of the Belkan Empire.

The war ultimately concluded with the collapse of the Belkan Empire. Near the end of the war, Belka's war efforts were undermined by nationalist revolutions, which saw the succession and independence of Recta and Gebet.

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