Japan's Prince Mikasa in 2015. (AP)

Prince Mikasa, who was a younger brother of former Japanese emperor Hirohito and who became an outspoken voice against Japanese barbarity during World War II, died Oct. 17 at a hospital in Tokyo. He was 100.

The Imperial Household Agency announced the death. Japanese media reports said he had been hospitalized since May, initially because of pneumonia.

Prince Mikasa, fifth in line to the Chrysanthemum Throne, is the uncle of the current emperor, Akihito. Akihito succeeded his father, Hirohito, whose 62-year reign encompassed World War II and continued until his death in 1989.

Prince Mikasa, also known as Takahito, was born in the Tokyo royal palace on Dec. 2, 1915. He was the youngest son of Emperor Taisho and Empress Teimei. He attended schools for nobility and royalty before enrolling in a military academy, where he graduated in 1936.

His service in a cavalry regiment took him to China, including the Japanese army’s headquarters in Nanking in 1943 and 1944, as a staff officer. He said he became increasingly shocked by the brutality of Japanese troops during World War II. At one point, he wrote in his best-selling memoir, he saw a film of Chinese prisoners “made to march on the plains of Manchuria for poison gas experiments on humans.”

Prince Mikasa, wearing the black skull cap, participates in military exercises with the 15th Cavalry Regiment, swimming mounts across a river in Tokyo on Sept. 3, 1936. (AP)

He reportedly tried to warn his oldest brother, the emperor, but it is unclear what effect he had. The emperor was a nationalist rallying point around which the military justified expansion throughout much of the Pacific and the sacrifice of millions of conscripts during the 1930s and 1940s. Shrouded in mystery were the emperor’s own views, whether he was in synch with the military or a closet pacifist and unwilling figurehead.

After the war, Prince Mikasa published a book that was deeply critical of Japanese military aggression and conduct, and he continued for decades taking bold public stands against those who wanted to justify Japanese imperialism during the war. He said the military scorned his brother’s desire for peace.

The prince spent his prime years studying Middle Eastern literature and culture at the University of Tokyo and later taught at several universities in the country. He appeared on radio and television cultural programs and was often spotted in public, even the subway.

He married Yuriko Takagi, and they had five children, three of whom died. A complete list of survivors could not be immediately confirmed.