Choi received a seven-year prison sentence but that was later changed to execution, to take place on August 18, 1945. On August 15, 1945, just three days before his execution date, Choi was spared execution by the liberation of Korea from Japan.
Following his release, Choi was recruited by the Kun Joon or Preparation Committee for Self Government in Seoul. In Seoul, he helped organize a student soldier party, the Student's Volunteer Group. The group later split into two groups, one advocating communism and the one Choi led that advocated democracy. In this role, Choi enrolled in a military language school, which later was to become the Korean Military Academy. At the school, Choi had a meeting with the superintendent, U.S. Army Major Reas, which led to Choi becoming one of the 110 founding fathers of the Korean Army.
In January of 1946, Choi was commissioned as a second lieutenant in the fledgling Republic of Korea army and was posted to the Fourth Infantry Regiment in Kwang-ju, Cholla Namdo Province, as a company commander. At that time, the local police were more powerful than the Army so military personnel were frequently beaten by the police for minor offenses. When Choi became a company commander, he taught his entire company Tang-soo so they could protect themselves. Choi hated having to teach his men a Japanese style of karate, so he began his research to create a new Korean martial art. According to Choi, "I began to teach karate to my soldiers as a means of physical and mental training. It was then that I realized that we needed to develop our own national martial art, superior in both spirit and technique to Japanese karate." Choi was then promoted to first lieutenant and transferred to Tae-jon to be in charge of the Second Infantry Regiment. While at his new post, he began spreading his new art not only to Korean soldiers but also to Americans stationed there. This was the first introduction to Americans of what would eventually become known as Taekwondo.