From On Desperate Ground: The Marines at The Reservoir, the Korean War’s Greatest Battle, by Hampton Sides (Doubleday, 2018), Kindle pp. 83-84:
This was the boomtown atmosphere in which Lee Bae-suk had grown up. Throughout the 1930s, Hamhung quickly became, in many respects, a Japanese city—organized, industrialized, modernized, militarized. Korea was living under what came to be called “the black umbrella” of absolute Japanese rule. The occupiers humiliated and exploited Hamhung’s citizens, often brutally, but they also sought to assimilate them—that is, to make them Japanese subjects, slowly eradicating all vestiges of Korean consciousness. As a boy in Hamhung, Lee was taught to bow toward the east, in the direction of the emperor. He prayed to Shinto gods, at Shinto shrines, kneeling in the shadow of red torii gates. At school, he and his classmates were required to recite the Pledge of the Imperial Subjects, promising to “serve the Emperor with united hearts.” Lee, like all citizens, had to forsake his Korean name and adopt a Japanese one. He learned the Japanese language and was forbidden to study Korean in school. The Korean anthem was not to be sung, the Korean flag not to be unfurled, traditional white Korean clothing not to be worn. People were even expected to give up Korean hairstyles, cutting off their braids and topknots.
Everywhere Lee looked, he saw examples of Japanese authority and expertise: Japanese teachers, Japanese civil servants, Japanese soldiers and tax collectors and cops. The mayor was Japanese. So was the provincial governor. Even the city itself was given a Japanese name: Hamhung became Kanko. The Japanese Kempeitai, which many Koreans came to call the “thought police,” tightened its hold on the city, stamping out dissent or expressions of Korean identity. The police organized the citizens into neighborhood associations, each one composed of ten families. These cells, designed to enforce compliance of Japanese laws, had a chilling effect on community relations, effectively turning Korean against Korean, requiring neighbors to spy on one another.
During the late 1930s, the industrial complex of greater Hamhung became an arsenal and a forge for Japan’s deepening war against China. Enormous quantities of explosives were manufactured there. After Japan attacked Pearl Harbor, operations at Hamhung expanded exponentially. Among other secret projects, Japanese physicists made early attempts to build an atomic weapon. Using uranium reportedly mined from the mountains around the Chosin Reservoir, they constructed a crude cyclotron, produced heavy water, and even began to develop a primitive atomic device.