In 1941, Sano assumed command of the IJA 38th Division. Early in the Pacific War, Sano and his division participated in the Battle of Hong Kong by invading Hong Kong Island on December 18, 1941. The British garrison surrendered on December 25, 1941. Sano's troops were reported to have committed atrocities against civilians in Hong Kong and British prisoners of war (Major General Takeo Itō, Sano's subordinate and commander of the 38th's Infantry Group, was tried and convicted after the war for war crimes). The division then transferred to Sumatra and participated in the Netherlands East Indies campaign.