In 1939, Saito was promoted to major general and appointed chief of the Military Remount Depot of the Guandong (Kwantung) Army. In 1941, he became chief of Military Remount Department of the War Ministry. Promoted to lieutenant general in 1942, Saito was appointed commander of the 43rd Division and given charge of the Northern Marianas District of Saipan and Tinian islands in April 1944.
The convoy carrying 7,000 men of the 43rd Division to the Marianas came under U.S. Navy submarine attack, and five of seven transports were sunk. Although most of the men were saved, they arrived at Saipan without their equipment. Saito also had little time to prepare for a U.S. invasion of the island. The main body arrived on 5 May and the remainder on 7 June, and U.S. forces invaded on 15 June. Saito had only his 43rd Infantry Division and the 27th Independent Brigade, in addition to naval personnel. Saito was junior to both Admirals Nagumo Chuichi and Takagi Takeo, who had their headquarters on the island, but few resources.
Saito's tactic of trying to defend at the water's edge was not successful, nor were the banzai night attacks against the U.S. beachhead. Overwhelming U.S. ground strength and naval and air support steadily reduced the defenders, who were driven into the northern part of the island. On 6–7 July, some 3,000 Japanese survivors mounted the war's largest banzai attack, which failed, and Saito committed suicide. That same day, U.S. Marine Major General Holland M. Smith declared the island secure. Kita Yoshito
Crowl, Philip A. United States Army in World War II: The War in the Pacific—Campaign in the Marianas. Washington, DC: U.S. Government Printing Office, 1960.; Morison, Samuel Eliot. History of United States Naval Operations in World War II. Vol. 5, The Struggle for Guadalcanal, August 1942–February 1943. Boston: Little, Brown, 1949.; Shaw, Henry I., Bernard C. Nalty, and Edwin T. Turnbladh. History of U.S. Marine Corps Operations in World War II. Vol. 3, Central Pacific Drive. Washington, DC: Historical Branch, G-3 Division, Headquarters, U.S. Marine Corps, 1966.; Sherrod, Robert L. On to Westward: The Battles of Saipan and Iwo Jima. Baltimore, MD: Nautical and Aviation Publishing, 1990.