The northernmost border of Japan's informal empire were the Mongol lands and Manchuria. For nearly a century this region lay on the borders between an expanding Russia and a retreating China and was a major area for Japanese ambitions on the Asian mainland. Japan's interest in the Mongol lands (including Manchuria) was spurred by general strategic, economic and ideological factors, and was justified by an academic discourse on geopolitics. Geo-political socialisation was carried out in respective areas and Mongol nationalism was drawn into these affairs. This paper examines the reasons for Japan's interest in these areas, and the arguments used to justify this expansionism.