In Republic of China Year 9 (1920), 25 years after the First Sino-Japanese War and henceforce the Japanese occupation of Korean peninsula and Taiwan islands, Zenichiro Kotobe obtained a travel permit from the Heilongjiang Military Governor’s Office to tour and investigate historical sites in the Han, Manchu, and Mongol regions of Northeast China (as well as the Siberian region of northern Heilongjiang / Amur river, then annexed by Russia for half a century).
In Taisho Year 13 (1924), Kotobe published Genghis Khan Was Minamoto no Yoshitsune, a work based on historical research and fieldwork conducted in Hokkaido, Kuye (Karafuto / Sakhalin), and mainland Northeast Asia. Written in a tone that oscillates between that of a hot-headed military youth 少壮派 (those who inspired the movement of the Young Turks) and a supposedly rigorous, cautious historian he articulated a theory that had long been popular among the Japanese people and had gained increasing traction at the turn of the 20th century: Yoshitsune—the legendary warrior and tragic hero of the late Heian period’s Minamoto-Taira war, younger brother of the first shogun of the Kamakura shogunate Minamoto no Yoritomo—did not commit seppuku at Takadate, but instead fled north in secret. He crossed the sea from Hokkaido, traveled westward into the Northeast Asian mainland, and ultimately became Genghis Khan, who then swept across the Eurasian continent.