察哈爾省
In 1936, Sakai was appointed commander of the IJA 24th Infantry Brigade, and was assigned to China with tensions rising in the prelude to the Second Sino-Japanese War. From 1937 to 1938, he was commander of the IJA 1st Independent Mixed Brigade under the Kwantung Army, which participated in Operation Chahar to seize Chahar Province from China. This brigade, with 744 vehicles, consisted of two tank battalions, a mechanized infantry regiment, field artillery, and engineering battalion, and was Japan's first mechanized brigade. However, during the Chahar campaign, the brigade was split into numerous small detachments for infantry support and was unable to achieve its full potential as an assault brigade. The small detachments suffered extensive damage and Sakai was in frequent conflict with the chief of staff of the Kwantung Army, Genera; Hideki Tojo over the strategy which the Kwantung army headquarters dictated. After the end of the operation, the brigade was dissolved and Sakai was transferred to rear echelon operations as commander of the IJA 7th Depot Division in 1938 and the IJA 109th Division in 1939.