The Korean War broke out on Jun 25th 1950 as North Korea waged a surprise attack and invaded South Korea. Although it was very disgraceful for South Korea to hand over its capital, Seoul, to North Korea only 3 days after the breakout of the Korean war, the Army was able to hold North Korea through effective stalling operations until U.S. and UN reinforcement arrived.

UN forces including South Korean Army formed a concrete defense at the Nakdong river. Thereafter, UN forces turned the table through the success of Incheon Landing Operation, and advanced North swiftly. However, an unexpected intervention from China forced UN and South Korea to retreat. Brutal, fierce battles over highlands along the 38th parallel continued until the two powers reached armistice.

The Korean War concluded on Jul 27th 1953 shortly after the Armistice Agreement, leaving catastrophic damages. Although there were only 8 divisions at the beginning of the Korean War, the ROK Army continued expanding and reinforcing the units during the war, and thus consisted of 3 corps and 18 divisions by the time of Armistice Agreement.