On Sunday, June 20, 1950 soldiers of the Democratic People’s Republic of Korea (DPRK) supported by heavy artillery fire crossed the 38th Parallel and attacked the Republic of Korea (ROK).
The DPRK troops were battle hardened from fighting the Japanese in World War II and were better trained than the ROK soldiers. The Communists had artillery and tanks, while the ROK troops had none.
The United States had token presence in South Korea, but they were ill equipped and unprepared for combat. After all, World War II was over and the military was in piece time mode. As always seems to be the case, the military was cut to the bone after the war.
By the end of June, Seoul had fallen, the ROK Army had been reduced by almost 2/3, the US troops were in full retreat.
On June 27, the United Nations passed a resolution urging member nations to provide military support and assistance to the ROK. By the signing of the Armistice in 1953 14 member nations of the UN would be fighting along side the US and ROK. The Chinese would come in on the side of the DPRK, and Russia would supply logistics support.
The war was on.
On July 5, a small detachment of the US Army 24th Division “Task Force Smith” attacked a superior force of DPRK troops. Without tank or artillery support the attack was beaten back with heavy casualties. The DPRK troops then pushed south and a several days later defeated the main body of the 24th Division inflicting heavy casualties.
The advance continued throughout August into September. Aircraft of the US Navy and Air Force provided air support and attacked DPRK forces and logistic assets. This had the effect of stretching the supply line to the breaking point. Which in turn allowed the ROK and US forces to hold at Pusan. Most of South Korea was in Communist hands, but that was soon to change.
On September 15, a combined US Army, USMC, and ROK force landed at Inchon. That force drove the DPRK forces back, and eventually recaptured Seoul. By mid October, ROK forces had captured Pyongyang, the North Korean capital.
At which point, China decided to enter the war.
The war went back and forth into the next year.
There is, of course, a lot more to the Korean War than I can cover in a blog post. One very good book is “This Kind of War” by T.R. Fehrenbach. The book covers the first year of so of the war in detail. It’s a must read for anyone who is interested in the history of the war that was known as a “police action.”