Adachi was born into an impoverished family, originally descended from samurai,[1] in Ishikawa Prefecture in 1890 (the 23rd year of the reign of Emperor Meiji, which is why his father, who had been a professional officer in the Japanese military,[1] chose the kanji for "23" to represent his given name "Hatazō").

Adachi had a reputation as a "soldier's general", sharing the miserable living conditions of his troops and welcoming open discussion with his officer and staff.

However, the IJA 51st Division, including Adachi and his senior staff, came under Allied air attack while en route from Rabaul to Lae, in the Battle of the Bismarck Sea.

His forces, suffering from malaria, heat exhaustion and malnutrition were rendered ineffective for the remainder of the war, despite Adachi's efforts to achieve some form of self-sufficiency by planting crops and giving priority in rations to the sick.

Although not personally involved in any of the atrocities mentioned, Adachi insisted on absorbing command responsibility for the actions of his subordinates during the military tribunal.

[1]: 109 In one of these, addressed to those officers and men of the Eighteenth Army who were then in the compound, he said:[4] "I felt it a great honour to have been appointed the C-in-C in November 1942, at a time when the issue of the day was to be settled, and posted to the point of strategic importance in order to ensure that the tide of war moved in our favour.

However, notwithstanding the fact that my officers and men did their best in the exceptional circumstances, surmounting all difficulties, and that my superiors gave the utmost assistance, the hoped-for end was not attained, because of my inability.

I have demanded perseverance far exceeding the limit of man's endurance of my officers and men, who were exhausted and emaciated as a result of successive campaigns and for want of supplies.