He returned to Korea when he was five years old, and spent a short time as an infant in Sinuiju, North Pyongan Province, and then grew up in Gongju, Chūseinan-dō.

After the North Korean invasion of South Korea proved to be too much, he was part of the effort to conduct a tactical withdrawal to the Naktong River.

The attempt was largely unsuccessful and General Yu's requests for US armor support were not always met but crucial to the retaking of the hill.

[2] Captured Inmin-gun documents showed that Kim Il Sung considered the hill to be a crucial strategic point and vital to winning the war but failed repeatedly in his attempts.

ROK Army Chief of Staff Chung Il Kwon wanted a Korean Unit to be the first to enter Pyongyang so he had Maj. Gen. Yu sent a regiment to take the city.

[2][4] In late October 1950, Maj. Gen. Yu was promoted to be Korean Army Deputy Chief of Staff and left for Seoul, leaving ROK II Corps to Brig.

The President of the United States of America, authorized by Act of Congress, 20 July 1942, takes pleasure in presenting the Legion of Merit, in the Degree of Officer to Brigadier General Yu Jae Hung, Korean Army, for exceptionally meritorious conduct in the performance of outstanding services to the Government of the United States from 25 June to 31 August 1950.

[6] The President of the United States of America, authorized by Act of Congress, 20 July 1942, takes pleasure in presenting a Bronze Oak Leaf Cluster in lieu of a Second Award of the Legion of Merit, In the Degree of Officer to Lieutenant General Yu Jae Hung, Republic of Korea Army, for exceptionally meritorious conduct in the performance of outstanding services to the Government of the United States from 28 January to 8 May 1952 and from 23 July 1952 to 1 February 1953.