During World War II, he alternately served as the chief of the Army General Staff's Operations Section and Secretary to Prime Minister Hideki Tojo.

Upon his promotion to colonel and chief of the operations section of the Army General Staff in 1941, Hattori played a key role in planning the Japanese conquest of Western territories during the early years of the Pacific War.

He subsequently remained in this position until a conflict with the Army's Military Affairs Bureau resulted in his transfer to a regimental command in China.

[10] His planned assassination attempt was to precede a National Safety Agency coup in which former military officers, many of whom had been removed in the postwar purge, would seize control of the government.

[12] Tsuji convinced Hattori to abort the alleged coup attempt because Yoshida belonged to the conservative Liberal Party, which was then definitively disavowed as the result of a withdrawal of US financial support.