After World War II, Korea was divided into two occupation zones along the 38th parallel (akin to the division of Germany). With the Soviet Union overseeing of the North and involved in providing arms, supplies and political advice to Kim Il-sung and the Communist forces in the North, the US became belatedly involved in the non-communist (but dictatorial) South to "fight communism."
After a buildup of arms and tensions, on June 25, 1950 the North Korean People's Army (NKPA) rumbled across the 38th parallel in Soviet tanks, and pushed the southern Republic of Korea (ROK)'s forces south to the very southern tip of Korea, at Pusan. The US sucessfully convinced the UN to intercede with a largely US force1. An amphibious landing by General Douglas MacArthur and his X-Force Marines at Inchon allowed the NKPA to be enveloped by a pincer movement and crushed. Seoul was retaken and the ROK forces moved North.
MacArthur's forces crossed the 38th parallel and pursued the NKPA deep into North Korea. This brought the People's Republic of China (PRC) into the fight, as they feared a US assault against Manchuria. The PRC army forced the UN and ROK forces back to the 38th parallel. MacArthur retrenched, and later forced a second assault into North Korea, against the explicit instructions of President Harry S Truman.
MacArthur was releived of command for violating orders, and his successor entrenched heavily at the 38th parallel.
The resulting stalemate dragged on until a peace treaty was signed at Panmunjom in 1953. Even then the tensions remained, and the demilitarized zone (DMZ) still stands2 as a reminder.
The United States still has 37,000 troops stationed in South Korea.
1. Non-korean UN forces makeup: 300,000 from the US; 14,000 from the UK, 6,000 from Canada, 5,000 from Turkey, 2,000 from Australia, and many UN nations with 1,000 to 1,500. Luxembourg sent 44.
2. According to November 2000's Discover magazine, a Korean scientist has proposed turning the DMZ into a huge
wildlife preserve, but he faces an uphill battle convincing the two Korean governments.