Asaka was promoted to colonel in 1925 and to major general in 1930 and then taught at the Staff College. In 1933, he received a further promotion to lieutenant general and assumed command of the Imperial Guards Division in 1933. Asaka became a member of the Supreme War Council in 1935. He used his influence as a member of the Imperial Family to further the interests of the military faction in the Japanese government.
As the Sino-Japanese conflict in Manchuria escalated in November 1937, Asaka was assigned the position of deputy commander of the Center China Area Army Group, which was driving on the then–Chinese Nationalist capital of Nanjing (Nanking). The commander, General Matsui Iwane, became ill, and as a result Asaka led the final assault on Nanjing, which ended with the fall of the capital on 13 December 1937. In the ensuing weeks, Japanese troops under Asaka's command committed atrocities on Chinese civilians and soldiers alike. This event, known as the "rape of Nanjing," was reported to the world by foreign correspondents. Asaka returned to Japan and was promoted to full general in 1939. He held no further field commands.
After the war, Asaka was questioned by U.S. investigators, but as a relative of the emperor he was not charged with war crimes. In his later years, he was well known in Japan for his interest in golf. Asaka died on 13 April 1981 at Atami.
Brackman, Arnold C. The Other Nuremberg. New York: William Morrow, 1987.; Chang, Iris. The Rape of Nanking: The Forgotten Holocaust of World War II. New York: Viking Penguin, 1998.; Yamaoto Masahiro. Nanking: Anatomy of an Atrocity. Westport, CT: Praeger, 2000.