The Washington Naval Conference convened on 12 November and the Americans proposed to scrap virtually every capital ship under construction or being fitted out by the participating nations. Mutsu was specifically listed among those to be scrapped even though she had been commissioned a few weeks earlier. This was unacceptable to the Japanese delegates; they agreed to a compromise that allowed them to keep Mutsu in exchange for scrapping the obsolete dreadnought , with a similar arrangement for several American dreadnoughts that were fitting out. Mutsu was commissioned on 24 October 1921 with Captain Shizen Komaki in command. Captain Seiichi Kurose assumed command on 18 November and the ship was assigned to the 1st Battleship Division on 1 December. Mutsu hosted Edward, Prince of Wales, and his aide-de-camp and second cousin, Lieutenant Louis Mountbatten, on 12 April 1922 during the prince's visit to Japan. On 4 September 1923, Mutsu loaded supplies at Uchinoura Bay, Kyushu, for the victims of the Great Kantō earthquake. With her sister Nagato, she sank the hulk of the obsolete battleship on 7 September 1924 during gunnery practice in Tokyo Bay, in accordance with the Washington Naval Treaty. Captain Mitsumasa Yonai, later Prime Minister of Japan, assumed command on 10 November. The ship was transferred to the reserve on 1 December 1925. Mutsu served as flagship of Emperor Hirohito during the 1927 naval manoeuvres and fleet review. Captain Zengo Yoshida relieved Captain Teikichi Hori on 10 December 1928. On 29 March 1929, the ship was assigned to Battleship Division 3, together with three light cruisers. Mutsus anti-aircraft armament was upgraded during 1932. Upon completion, she was assigned to Battleship Division 1 of the 1st Fleet, and again served as the Emperor's flagship during the annual maneuvers and fleet review in 1933. The ship was placed in reserve on 15 November and began her lengthy reconstruction. This was completed on 30 September 1936 and Mutsu rejoined the 1st Battleship Division on 1 December 1936. In August 1937, she transported 2,000 men of the 11th Infantry Division to Shanghai during the Second Sino-Japanese War. Her seaplanes bombed targets in Shanghai on 24 August before she returned to Sasebo the following day. On 15 November 1938, Captain Aritomo Gotō assumed command of the ship. Mutsu was placed in reserve from 15 December 1938 to 15 November 1939. She was refitted in early 1941 in preparation for war; as part