World War II divided Korea into a Communist, northern half and an American-occupied southern half, divided at the 38th parallel. The Korean War (1950-1953) began when the North Korean Communist army crossed the 38th Parallel and invaded non-Communist South Korea. As Kim Il-sung's North Korean army, armed with Soviet tanks, quickly overran South Korea, the United States came to South Korea's aid. General Douglas MacArthur, who had been overseeing the post-WWII occupation of Japan, commanded the US forces which now began to hold off the North Koreans at Pusan, at the southernmost tip of Korea. Although Korea was not strategically essential to the United States, the political environment at this stage of the Cold War was such that policymakers did not want to appear "soft on Communism." Nominally, the US intervened as part of a "police action" run by a UN (United Nations) international peace- keeping force; in actuality, the UN was simply being manipulated by US and NATO anti-Communist interests.
With the US, UN, and South Korean (ROK) forces pinned against the sea at Pusan, MacArthur orchestrated a daring amphibious assault on Inchon, a port on the western coast of Korea. Having made this landing, MacArthur crushed the North Korean army in a pincer movement and recaptured Seoul, the capital of South Korea. Instead of being satisfied with his rapid reconquest of South Korea, MacArthur crossed the 38TH Parallel and pursued the North Korean army all the way to the northernmost provinces of North Korea. Afraid that the US was interested in taking North Korea as a base for operations against Manchuria, the People's Republic of China secretly sent an army across the Yalu River. This Chinese army attacked the US/UN/ROK forces. Only after the appointment of Lt. General Matthew Ridgway as commander of ground forces did American morale improve and the initiative begin to swing against the Chinese Communists.
Although President Truman hoped to end the war quickly and pressed MacArthur to be more tactful, the brilliant strategist went against presidential orders and continued spouting incendiary lines about his hopes to reunify Korea. After gaining the support of the Joint Chiefs of Staff (JCS), Truman relieved MacArthur of command. The move was extremely unpopular in America; MacArthur was perceived as a popular war hero. Only the support of the JCS saved Truman from impeachment after the firing.
Ridgway took MacArthur's command and held off the Communists with strong fortifications and entrenchments just north of the 38TH Parallel, sending occasional offensives against the Iron Triangle, the Communists staging area for attacks into South Korea. Peace negotiations dragged on at Kaesong, then moved and continued to drag at Panmunjom through 1951 and 1952. The US tried using strategic bombing to intimidate the Communists into negotiating a peace treaty, but they wouldn't budge, particularly on the issue of POW (Prisoner of War) repatriation. Neither side wanted to appear weak, and so the talks went on, occasionally breaking down for months. Only after Eisenhower, who was a war hero and was unafraid of Republican criticism (since he himself was a Republican), became President, could the US make substantial concessions to the Communists. In 1953 a peace treaty was signed at Panmunjom that ended the Korean War, returning Korea to a divided status essentially the same as before the war. Neither the war nor its outcome did much to lessen the era's Cold War tension.
Korea and its neighbor Manchuria had been of great importance to the USSR, the PRC (People's Republic of China) and Japan since the nineteenth century. Following this tradition, after World War II the USSR made an attempt to occupy Korea. Not wanting the Soviets to grab too much territory, the US occupied the southern half of Korea, south of the 38th Parallel. Much as it had in Germany just after World War Two, these two occupations set the status quo: North Korea, that area of the Korean peninsula north of the 38th parallel became Communist, while South Korea was the province of a nationalist, anti-communist government.
The Korean War erupted on June 25, 1950 in the middle of the burgeoning Cold War, an international struggle between the US and the USSR for world domination of their competing ideologies, Democracy/Capitalism versus Communism. While the Soviet Union never got directly involved in the fighting, it did supply North Korea with weapons and supplies. The US, on the other hand, did commit its own troops as part of a UN international-peace keeping force. In reality, the UN force was in name only; the troops were made up of almost entirely American forces, with some American allies. The Korean War was the first instance that it became clear that the UN could be used by the US as a foreign policy tool.
It is somewhat surprising that only a few years after letting enormous China turn Communist without getting seriously involved, as well as watching Eastern Europe fall under the "iron curtain", the US would then become embroiled in an Asian land war over the fate of strategically insignificant Korea. The Korean War thus represented an important shift in US Cold War policy. By 1950, a loss to communism anywhere was thought of as a loss everywhere. The beginnings of the later Domino Theory were already present in an early form.
The US got involved in Korea to save face and to appear strong against communism, not because Korea was vital to American interests. Somewhat ironically, South Korea was only a sham democracy under Syngman Rhee, who was really just as tyrannical as North Korea's Kim Il-Sung. Once again, this set a Cold War pattern for the US: support of anti-communists who were quite blatantly dictators themselves, and the tautological justification of that US support for the simple reason that these dictators were anti-communist.
One of the significant results of the Korean War was that it gave the US reason to increase its military expenditure four-fold. Under Truman, military expenditure increased rapidly, laying the foundations for the so-called military industrial complex that existed throughout the Cold War. Perhaps on a more positive note, it was during the Korean War that black and white troops were first integrated in the US army, an important step on the road to civil rights. The Korean War also strengthened the US relationship with Britain, which sent troops for the UN peacekeeping force. Finally, it was during the Korean War (and partially because of it) that the Democratic monopoly of the Presidency, going back to before World War II, finally ended with the election of Eisenhower.
Another result of the Korean War was the ascendance of the People's Republic of China onto the world stage. Fighting against the US, China received aid from the Soviets, helping them to become a major military power. The US had proved the fulcrum in both World War One and World War Two, with its forces providing the force needed for its European allies to overcome its enemies. The Chinese forces, however, fought the US to a standstill, as represented by the reinstitution of the 38th parallel as the dividing line between North and South; in fighting against the US in the first war the United States entered and did not win, China established itself as a power to be reckoned with, and a communist power at that.
The Korean War also proved the tenacity and skill of the Communist Asian militaries, something that would be reaffirmed by the Vietnam War in the 1960s. In fact, remarkable similarities exist between the Korean War and the Vietnam War; from the US support of a dictatorial and corrupt anti-communist regime to its conception of communism as a monolithic entity, under which all communist nations were necessarily allies, rather than individuals to be dealt with separately. However, though those parallels, Vietnam era policy-makers did not apply the lessons of the Korean War to the Vietnam War. Rather, they did not seem to recognize those lessons as lessons at all, and repeated in the Vietnam War many of their previous mistakes.
The Korean War also showed the impact a single individual can have on history. General MacArthur's brilliant strategies, willfulness, egomania, and refusal to obey orders dramatically influenced the outcome of the war, in both positive and negative ways.
Finally, the Korean War demonstrated the new terms of the new post-WWII era, and showed how difficult it would be to fight a limited war under those terms. Although the United States attempted to keep the war on a very small scale, it quickly snowballed out of proportion, involving China, at times seeming as if it might become a World War III. Looked at another way, though, the Korean War can be considered a success: although the war did at times get out of hand, the US and the USSR were able to avoid direct confrontation, especially since the USSR fought mainly by proxy. Perhaps most importantly of all, though it was fought just five years after Hiroshima and Nagasaki were bombed, the Korean War was not an atomic war, avoiding both the possibility of immediate nuclear holocaust (since the USSR by then had its won bombs) and setting a pattern that would continue throughout the Cold War.