At the time of the
Coup d'état of December Twelfth, he was the Deputy Chief of Staff of the Korean Army. After the kidnapping of the Chief of Staff
Jeong Seung-hwa, he automatically rose to the top position in the army chain of command and took in charge of the whole government forces. However, he was too indecisive to make a quick decision of summoning forces to quell the rebels. He was then easily deceived by the rebel leader
Chun Doo-hwan that Yoon ordered a withdrawal of the government force, which gave rebels the opportunity to advance without resistance and eventually take control of the entire
Seoul. Unlike most other generals on the government side, who were either demoted or discharged after the incident, Yoon's incompetence during the coup ironically won him favour of Chun Doo-hwan, who promoted him to become the commander of the I Corps. ==Later life==