He graduated from the 26th class of Imperial Japanese Army Academy in 1914 and served as a junior officer with the IJA 5th Cavalry Regiment.

Assigned to Hailar, in Inner Mongolia which was also the site of an extensive Japanese static military fortification system, it was primarily a reserve and training garrison force.

[1] However, as the war situation continued to deteriorate for the Japanese military in the Solomon Islands, Sasaki was reassigned to command the Southern Detachment in May 1943.

After fighting an effective, but ultimately unsuccessful delaying campaign, his forces retreated to Kolombangara, only to be bypassed and left to starve, with little chance of reinforcement or resupply.

In the Battle of New Georgia his outnumbered forces stood off nearly four Allied divisions and successfully evacuated 9,400 men to fight again.