Both north and south korea were together and they had two leaders that co-owned the parallel as a whole but stayed to one side. Neither leader would stick to the agreement and would often intrude the other side which caused conflict between the two.
The north infiltrated the south with a surprize attack. " The 75,000-strong North korean army, well trained and armed by their soviet and chinese communist allies , made short work of the poorly prepared south korean defenders."
The rush of north korean soldiers scared the US and alarmed them because south korea was there wall that was stopping communism from spreading across asia. "Two days after the invasion began, President Truman announced that the US was to commit its military forces to protect south korea."(The Week)
After north and south korea had a bloody back and forth a truce was made between the two sides. "The two sides stopped short of a peace accord meaning they are technically still at war" although it has been years and years since then.(The Week)