At the beginning of the Sino-Japanese War in 1937, Adachi commanded the 37th Infantry Division. As chief of staff of the North China Area Army, Adachi played a prominent role in defeating Chinese forces during the chugen Operation of May 1941. He was promoted to lieutenant general in 1940.
Following the death of Lieutenant General Horii Tomitaro in November 1942, Adachi was appointed commander in chief of the newly formed Eighteenth Army at Rabaul, which was designated for the defense of New Guinea and numbered at peak strength some 140,000 men. The Eighteenth Army's first full-scale landing attempt on New Guinea was shattered in the March 1943 Battle of the Bismarck Sea, when U.S. air attacks sank all of Adachi's eight transports, resulting in the loss of 3,000 men and a great amount of supplies and equipment.
Allied air superiority forced Adachi to bring his troops ashore in scattered small formations along more than 400 miles of the north New Guinea coast. A lack of transport and air cover inhibited the concentration of Eighteenth Army forces and led to the deaths of many of Adachi's men from hunger and disease.
When on 22 April 1944 Allied forces went ashore at Hollandia and Aitape, key strategic points on the north-central New Guinea coast, Adachi skillfully assembled his remaining 60,000 troops and moved them 300 miles overland to contest the Allied forces. Adachi ordered an all-out attack on 10 July 1944, and his forces successfully broke through the Allied front lines in place, but the early attacks soon lost momentum and Adachi had to withdraw. In the struggle for Aitape, Adachi lost more than 10,000 men. Thereafter, the Eighteenth Army simply struggled to survive Australian army mopping-up operations. On 13 September 1945, Adachi surrendered with only about 10,000 of his original force alive.
Although Adachi was an officer of great personal integrity and fighting ability, especially in adversity, his offensive tactics have been called into question and contrasted with those of his superior, General Imamura Hitoshi, commander in chief of the Eighth Area Army at Rabaul. Adachi's repeated attempts to attack superior Allied forces led to unnecessarily heavy casualties. In contrast, Imamura on New Britain Island prepared his men for sustained defensive operations and followed a "force-in-being" strategy throughout the remainder of the war.
After the war, the Australian government prosecuted Adachi for war crimes, specifically the mistreatment of Allied prisoners of war at Rabaul. He was convicted and was sentenced to life imprisonment. Adachi committed suicide in prison at Rabaul on 10 September 1947. Tohmatsu Haruo
Drea, Edward. In the Service of the Emperor. Lincoln: University of Nebraska Press, 1998.; Komatsu, Shigeo. Ai no tosotsu Adachi Hatazo (Beloved commander: Adachi Hatazo). Tokyo: Kojinsha, 1989.; Long, Gavin. The Final Campaigns. Canberra: Australian War Memorial, 1963.