After his compulsory service he was recommended for Officer School, and graduated from the Republic of Korea Military Academy in 1972 in the Self-Propelled Artillery branch.

Prior to assuming the position of Chairman of the Joint Chiefs of Staff in 2008, he was Commanding General of the First ROK Army.

In September 2009, he replaced Lee Sang-Hee as the 42nd Republic of Korea Minister of National Defense.

On 25 November 2010, he stepped down from his post as Minister of Defense in the wake of criticism he received for his handling of the shelling of Yeonpyeong.

[1] The next day it was announced that his successor would be Kim Kwan-Jin, his JCS Chairmanship predecessor.