Kim graduated from Belmont High School in Los Angeles, and proceeded to Los Angeles City College. After a year, thinking the advanced education would not do any good, he dropped out. He tried various jobs, but his racial background did not allow him to stay with one job for long. After World War II broke out, he applied to U.S. Army, but was turned away for the same reason.
In January 1941(?), though, he was drafted by the Army, after the U.S. Congress passed a law that allowed the Army to draft Asian Americans. This was three months before his father died.
100th Battalion was sent to North Africa to assist in the war in Europe, but initially the U.S. Army had no plan on how to utilize the unit. By its own request, it was sent to the front and joined the war in Italy. There, Kim's map-reading skills and determination led to success in many battles and some "impossible missions". His skills started to gain recognition.
An anecdote from the Battle of Anzio is well-known. In this critical battle, the Allies needed to determine the locations of German tank units. Kim volunteered to capture German soldiers to gain intelligence information, and he and Private First Class Irving Akahoshi crawled into German territory and captured two German soldiers in the daytime, while the enemy rested for their evening watch. The information they gathered from the prisoners helped determine that there was not a tank unit in the path they were considering to break through. The Allies was able to break Gustav Line and liberate Rome.
The battalion was then sent to France, where Kim participated as an operations officer. He fought in battles that liberated the towns of Bruyères and Biffontaine, but was severely wounded by a gunshot from an enemy soldier in Biffontaine, and after a while, returned to Los Angeles for a 6-month break in late 1944. When he was about to return to Europe, the European part of the war was over.
The Army let all Korean-heritage soldiers, and anyone who could speak at least a word in Korean, work in the Army Security Agency. Kim was no exception, but he wanted to fight. At his request, he was sent to East Asia, and by pretending not to know any Korean and with the help of people he had known from World War II, he was able to join the infantry. This was the first time he had ever been to Korea.
He was assigned to the the 31st Infantry of the 7th Infantry Division in April, 1951 as the Chief Intelligence Officer, under William J. McCaffrey, who scouted him. Kim worked not only as an intelligence officer, but also virtually as an operations officer, by the request of McCaffrey. Kim rescued many U.S. and Korean Army troops in several battles.
The battalion was previously known to be incompetent, but with the arrival of Kim, it won nearly every battle it participated in, including the battles of Kuman-mountain, Tabgol, Keumbyung-mountain, and Suahn-mountain. The 31st Infantry played a major role in stopping the Chinese troops, and pushing them back above the 38th parallel. Kim's unit was the very first unit that crossed the parallel. The 8th Infantry Division of U.S. Army redrew the map every day to reflect the changes, but these maps recorded only the locations of bodies that were bigger than or equal to regiments, which are groups of battalions. However, the map from May 31 1951 included the location of Kim's battalion. Kim played a major role in shaping the current border between the Koreas.
During Operation Piledriver in August, after a battle in which his unit proceeded to the north of Kimhwa, his unit was mistakenly bombarded by the 555th Artillery Battalion because it seemed too far north to be friendly. Kim was seriously injured in the friendly fire incident. He was lucky enough to be saved by doctors from Johns Hopkins University who were in Tokyo. He made it back to Korea after two months.
Upon his return, McCaffrey let him command the 1st combat battalion of the regiment, which made him the first-ever officer from an ethnic minority to command an Army combat battalion in U.S. history. After fighting for nearly a year, Kim left Korea in September 1952.