'Truman's troubles in Asia exploded on the Korean peninsula. In the wake of World War II, Korea had been partitioned at the 38th parallel, with the Soviets supporting a communist regime north of that boundary and the Americans a non-communist one in the south. On June 25, 1950, North Korea launched a surprise invasion of South Korea. The United Nations immediately condemned North Korea, while Truman and his advisers in Washington discussed the American response. Certain that the Soviet Union lay behind the invasion, they reasoned that failure to act would lead U.S. allies to question America's commitment to resist Soviet aggression. Truman resolved not to repeat the mistake of Munich, where the European powers appeased and condoned Hitler's expansionism. Scholars now know that the invasion was the brain-child of North Korean leader Kim Il-sung and that Stalin acceded to it only after making clear that the Soviets themselves would not become involved militarily and that Mao provide ground troops. Ultimately, the Soviets did provide the North Koreans with air support.
Truman ordered the American military, under the direction of General Douglas MacArthur, to intervene. The first U.S. troops did little to stop the onslaught as North Korean forces made rapid progress in their march down the peninsula. By August, the Americans were holed-up in a defensive perimeter on the southeastern tip of South Korea. MacArthur launched an audacious and risky counter-attack the following month that featured an amphibious landing behind enemy lines at Inchon on the western coast of South Korea, near the capital of Seoul.
MacArthur's gamble worked; American forces rapidly drove the North Koreans back to the border at the 38th parallel. MacArthur then received permission from the Truman administration to cross the border to secure the final defeat of North Korea and the reunification of the country. The danger, though, was obvious. The Soviet Union and China both bordered North Korea and neither wanted an American-led military force, or an American ally, on their doorsteps. In mid-October, meeting with the President at Wake Island, MacArthur told Truman that there was "very little" chance of Chinese or Soviets intervention. At the same time, however, the Chinese warned American officials though third-party governments that they would enter the war if the United States crossed the 38th parallel.
Disregarding these warnings, American forces pushed northward throughout October and into November 1950, coming to within several miles of the Chinese border. The Chinese entered the battle in late November, launching a massive counter-attack that threw the Americans back south of the 38th parallel; an American response in the spring of 1951 pushed the front north to the 38th parallel, the status quo anti-bellum. A brutal and bloody stalemate ensued for the next two years as peace talks moved forward in fits and starts.
American involvement in Korea brought Truman more problems than successes. After General MacArthur publicly challenged the administration's military strategy in the spring of 1951, Truman fired him. MacArthur returned home a hero, however, and Truman's popularity plummeted. Against the backdrop of McCarthyism, the failure to achieve military victory in Korea allowed Republicans to attack Truman mercilessly. Indeed, the war so badly eroded Truman's political standing that the President's slim chances of winning passage of his "Fair Deal" domestic legislation disappeared altogether.
Despite these setbacks, Truman's decision to stand and fight in Korea was a landmark event in the early years of the Cold War. Truman reassured America's European allies that the U.S. commitment to Asia would not come at Europe's expense - a commitment made more tangible in 1951 by increased American troop deployments to Europe and not Korea. The President thus guaranteed the United States to the defense of both Asia and Europe from the Soviet Union and its allies. Likewise, the Korean War locked in the high levels of defense spending and rearmament called for by NSC-68*).Finally, the American effort in Korea was accompanied by a serious financial commitment to the French defense of a non-communist Indochina. In a very real sense, Korea militarized the Cold War and expanded its geographic reach.'
By Alonzo L. Hamby who is Distinguished Professor of History at Ohio University and a biographer of Harry S. Truman.
Source:
http://millercenter.org/president/biography/truman-foreign-affairs
*) About 'NSC-68':
https://history.state.gov/milestones/1945-1952/NSC68
