Japanese Emperor Hirohito was long depicted as a naïve pacifist who reluctantly acceded to the wishes of his hawkish military leaders. But the newly released diaries of one of his closest aides suggest the emperor posthumously known as Shōwa thought war with the West was inevitable and was preparing for it earnestly two months before Japan attacked Pearl Harbor. Relatives of Admiral Saburō Hyakutake, who served as the emperor’s grand chamberlain and managed the royal household, deposited 25 of his diaries and pocket notebooks, along with memos he wrote, to the library at the University of Tokyo’s graduate schools for law and politics. They became available to the public in September. Hirohito escaped a war crimes trial based on the public premise that he had resisted ratcheting up...