After the establishments of the Democratic People’s Republic of Korea in the North and the Republic of Korea in the South, it quickly became clear that the hopes of a unified Korea were impossible due to the ideological differences. It was also evident that within a short amount of time one side would try to overtake the other. A war was inevitable, and it was only a question of who would make the first move.
On June 25, 1950, the North Korean People’s Army (KPA) with support from the Soviet Union and China crossed the 38th parallel and invaded the Ongjin Peninsula. The South Korean and American soldiers stationed at Ongjin were on weekend leave. This allowed the KPA to quickly close in on Seoul–the capital of the South– and take the city without any resistance in a matter of hours. The UN Security Council was quick to condemn North Korea’s invasion. However, after a few days, the KPA had almost captured the entire southern half of the peninsula.
The South Korean Army was backed by a United Nations Command force. US General Douglas MacArthur, the World War II hero of the Philippines and Supreme Commander of Allied Forces for the American Occupation of Japan, was given command of the entire Asian Theater. In the summer of 1950, the UN and South Korean forces were able to secure a defensive position around the city of Pusan at the bottom of the peninsula. This deterred the KPA’s advancement.
Once MacArthur was given command, the war for the Americans turned from defensive to offensive. MacArthur, who was rather full of himself, believed that it was his job to “liberate” the North from the communists. This was initially successful. MacArthur planned a large amphibious landing at the South Korean city of Inchon, which was a huge success. This boosted morale and allowed MacArthur’s army to push the North Koreans out of Seoul and eventually back over the 38th parallel.
However, as MacArthur headed north in the direction of the Yalu River–the river that formed the border with North Korean and communist China– the Chinese began perceive MacArthur’s motives as an “armed aggression against Chinese territory.” Mao Zedong, the communist dictator of China, sent troops to the Yalu Valley and warned MacArthur to keep clear of the border unless he wanted a full-scale war.
Get the Party Started 
Nice pic of Dougš
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why was Korea divided in the first place? i understand Inchon was miraculous because the timing depended on the tides. and, Mac had to be relieved of command, by Truman.
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As the Pacific War started to end, the United States and the USSR drew a line on the 38th parallel and agreed to do separate occupations on either side. This is discussed in the previous post titles Kim Il-Sung and the Korean War.
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