Korean War
After the end of WWII the Korean peninsula was separated into occupation zones between the US and the USSR with the US occupying the south and the USSR occupying the north. This leads to the north being communist and the south being a democratic nation.
However with such close tensions war broke out. North Korea quickly invaded South Korea conquering most of the land before in 1950 the US would send aid and troops to help support South Korea along with support from the UN. The US quickly drove back the North Korean army and with a major offensive placed behind their lines forced the North Koreans into a retreat.
However once the US drove through most of North Korea, China entered the war again pushing back the front lines back to the 38th parallel. After this push back the war quieted down before eventually ending with a demilitarized zone declared along the border between North and South Korea.