In September 1919, he was assigned to the Siberian Expeditionary Army, and served in the Japanese intervention in Siberia during the Russian Civil War, based in Manzhouli. Sasaki's role was to serve as a liaison officer between Japanese and Chinese forces under the Sino-Japanese Military Agreement of May 1918, and later, to help provide military assistance to White Russian Cossack leader Grigory Mikhaylovich Semyonov. After Japan cut off assistance to Semyonov in line with the Gongota Agreement of 1920, Sasaki's immediate superior ordered him to help Semyonov escape east to Primorskaya Oblast, contrary to the army's policy. Having completed this mission, the unknowing Sasaki reported to the Japanese command in Vladivostok, upon which he was reprimanded for aiding Semyonov, whose presence in the region could upset Japan's efforts to support the Far Eastern Republic neutral buffer state. Thereafter, Sasaki was effectively demoted, given a job with no duties in Vladivostok, and in June 1921, made to return to Japan to serve as a company leader (an inferior post that he had already served in after graduation from the Army War College). Sasaki wrote that this experience affected him greatly, and that he seriously considered quitting the military, but decided to remain following advice from his friend Sakai Takashi, who then worked in the China Department at the Imperial Japanese Army General Staff Office. In December 1921, he was appointed to work in the China Department, marking his return to active involvement in Chinese affairs.