In Kyoto, Japan, Choi considered learning boxing, but a fellow Korean, Kim Hyun Soo, a Shotokan karate instructor, convinced Choi to train with him at Dong Dai Sa University. After two years of concentrated training, Choi earned his first dan in Shotokan karate from Kim. To improve his educational opportunities, Choi later moved to Tokyo and entered the Dong A Business High School which allowed him to later enter the Law School of Choong Ang University. While at the university, Choi continued his Shotokan studies under master Guchin Funagoshi. After earning his second dan in Shotokan, Choi and his friend Byung In Yoon taught Shotokan karate on the roof of the Tokyo YMCA. Choi recalls that, during this time, he struck or kicked every lamppost in the city to make the copper wires overhead vibrate. Choi said that:
"I would imagine that these were the techniques I would use to defend myself against the wrestler, Mr. Hu, if he did attempt to carry out his promise to tear me limb from limb when I eventually returned to Korea."
Whereas Oyama stayed in Japan, Choi returned to Korea. With the outbreak of World War II, he was forced to enlist in the Japanese army as a student volunteer on October 20, 1943. He sent to basic training at Seoul National University. While at his post in the 42nd Unit of the Pyongyang Division in Korea, Choi became involved with a group of about 30 Korean student-soldiers that had decided to escape to the Baek Mountains located on the Manchuria-Korean border, an incident known as the Pyongyang Student Soldiers' Incident or the Korean Independence Movement. The escape plan failed after it was discovered by the Japanese military. Choi was convicted as the planner and was interned at a Japanese prison during his eight-month pretrial examination. While in prison, to alleviate the boredom and keep physically fit, Choi practiced his martial art in the solitude of his cell. In a short time, his cellmate and the jailer became his students. Eventually, the prison courtyard became a gymnasium.