According to "Koraishi" (The History of Goryeo) and "Genshi" (The History of the Yuan Dynasty), Mongol high officials were worried about soldier shortage and insisted on conquering the Southern Sung Dynasty first, but Goryeo (later King Chungnyeol) persistently requested them to invade Japan from the eastern route via Goryeo, and thus it was so decided.
『高麗史』及び『元史』によれば、高麗の（のちの忠烈王の）執拗な要請があり、蒙人の高官は兵力不足を懸念して南宋攻略を先にすべきと主張したが、高麗を経由する東路からの日本侵攻が決定されたとされる。