However, the Japanese army suffered a ruinous defeat at the hands of the combined Soviet-Mongolian forces and Yasuoka was relieved of command in July 1939 and his detachment was dissolved.

In July 1942, Yasuoka agreed to accept the post of governor of Surabaya in Japanese-occupied Java, Netherlands East Indies.

During his time in Surabaya, he learned enough of the Javanese language to be able to make a speech in public, and was known for visiting school and government offices without notice.

He attempted to employ as many native Indonesians in government positions as possible, both from a practical standpoint, but also from a belief that the Netherlands East Indies should be freed from colonial rule.

[3] He held the post until the surrender of Japan in August 1945, whereupon he was arrested by Dutch colonial authorities and tried before a military tribunal where he was declared guilty of unspecified war crimes.