At least nine Korean masters trained in Japan: Yong-Shul Choi, Geka Yung, Hyung-Ju Cho, Won-Kuk Lee, Pyong-Chik Ro, Hong-Hi Choi, Yong-i Choi, Ki-Whang Kim, and Pyung-In Yun. Yong-Shul Choi claims to have trained for many years in Daito-ryu Aikijutsu under Sokaku Takeda, although his claims are not recognized by the followers of Morihei Ueshiba, the founder of Aikido. Choi later returned to Korea and taught Yusul (Jujutsu), which one of his students, Ji-Han Jae, later called Hapkido ("coordinated energy way"). The other eight Koreans trained in Karate-do. Geka Yung was the head instructor of the Kanbukan ("Korean martial arts hall") in Japan, which was later renamed the Renbukan ("training martial arts hall") under Norio Nakamura. Hyung-Ju Cho moved to Japan, changed his name to Neichu So, and trained in Goju-ryu Karate-do under Chojun Miyagi in high school, becoming a karate-do instructor in 1939. According to Hancock, Won-Kuk Lee learned Shotokan karate-do while attending school in Japan. Pyong-Chik Ro studied at a Japanese university during the Second World War, during which time he also studied under Gichin Funakoshi and earned his first dan (black belt rank) in Karate-do before returning to Korea in 1944. Hong-Hi Choi and Yong-i Choi both went to Japan in the late 1930s and later became famous martial art masters (see chapter 11). Ki-Whang Kim began Judo in 1931 and earned his first dan from the Kodokan five years later. He went on to study Karate-do at Nihon University in Tokyo, where he captained the team and was nicknamed "typhoon." He later spent two years "studying Kempo and kung-fu in China," probably as one of the draftees of the Japanese Army. Pyung-In Yun was raised in Manchuria and studied quan-fa there before also attending college at Nihon University. He trained there with one of the faculty members, Kanken Toyama (1888-1966), who also happened to be the founder of Shudokan Karate-do. Before Yun returned to Korea, Toyama recognized him as a fourth dan in his style.
Ki-Whang Kim (1920-1993) was able to begin Judo in Korea in 1931, despite the Japanese ban. Eventually the underground nature of the martial arts in Korea changed when the Japanese lifted the ban on martial arts in 1943 so it could fulfill military requirements during World War II. Judo and "Juken-jutsu" (bayonet art) began to be taught in 1941, and by 1943, Karate and kung-fu were officially introduced to Koreans. For the two years before the surrender of Japan, the martial arts enjoyed a new popularity in Korea. A select few still practiced Taekkyon, Subak, Kong-soo, and Hwa-soo but they did not share their expertise with the public. The actions of Korean martial artists in Korea in those days remains largely unknown.