With the start of the Second Sino-Japanese War, Ise was assigned to patrols off the southern Chinese coast. Yamaguchi was promoted to rear admiral on 15 November 1938. From December, he was the chief-of-staff of the IJN 5th Fleet and from November 1939 was on the staff of the IJN 1st Fleet. in January 1940, he became commander of the 1st Combined Air Group, and in this capacity directed a saturation bombing campaign in central China through 1940, including the Bombing of Chongqing.