Russia and the United States began their relationship as allies when Russia disregarded the non aggression pact they signed with Germany in 1939, effectively sealing Germany’s fate in World War II.
Korea, like Germany, had been occupied by Soviet and United States forces at the end of World War II. Korea was split in half via the 38th parallel after the Japanese Empire fell near the end of the conflict. The soviets occupied the north side of the line and the Americans occupied the south. “By the end of the decade, two new states had formed on the peninsula. In the south, the anti- communist dictator Syngman Rhee (1875-1965) enjoyed the reluctant support of the American government; in the north, the communist dictator Kim Il Sung (1912-1994) enjoyed the slightly more enthusiastic support of the soviets.” (http://www.history.com/topics/korean-war) Many top decision makers in the US believed that the USSR was trying to spread communism throughout the world.
In April 1950, a National Security Council report recommended that the U.S. use military force to “contain” communist expansionism anywhere it seemed to be occurring. “The NSC-68 called for significant peacetime military spending, in which the U.S. possessed "superior overall power" and "in dependable combination with other like-minded nations." It calls for a military capable of: Defending the Western Hemisphere and essential allied areas in order that their war-making capabilities can be developed; providing and protecting a mobilization base while the offensive forces required for victory were being built up; conducting offensive operations to destroy vital elements of the Soviet war-making capacity, and to keep the enemy off balance until the full offensive strength of the United States and its allies can be brought to bear; defending and maintaining the lines of communication and base areas necessary to the execution of the above tasks; and providing such aid to allies as is essential to the execution of their role in the above tasks.” ( http://en.wikipedia.org/wiki/NSC-68) Non-intervention was not a viable option.
On June 25, 1950, the Korean War began when 75,000 North Korean People’s Army (NKPA) troops moved across the 38th parallel. Later that same day the United Nations demanded that North Korea withdrawal its forces and return to its borders. Kim Il Sung disregarded this and ordered his troops to press forward and by June 29, 1950, the capital of south Korean fell to the NKPA.
The United States geared up for a war against communism. Not more than a day after Seoul fell, the US had officially entered the war and had ground troops committed, an action backed by the U.N. At the beginning of the war the U.S. soldiers had little combat experience and they were in a foreign land. This was the reason for the mass casualties sustained by the U.S. “US Army casualties from June 25 through July 31, 1950 totaled 6,003:...