Rather than asking for a declaration of war from Congress, Truman opted to claim that he was sending ships and planes at the request of the UN. This allowed Truman, rather than Congress, to take credit for responding to the Communist threat. Indeed, Congress, the Press, and the Gallup Polls all responded very favorably to Truman's policies. One side-effect that few considered was the following: since the 7TH Fleet was barring the PRC from invading Formosa, the PRC transferred most of its army to Shantung province, a location from which it would be easy to quickly become involved in the Korean conflict.
The UN force that went to Korea was the first-ever "international peace-keeping force." But although its ideology of peace and non-aggression seem very positive, the peacekeeping force was in this case actually an instrument of the US. Only 16 countries actually sent men on the mission, and most of these were NATO countries, which were hardly neutral when it came to Communists. And the majority of the troops by a good margin were American: while 5.7 million American troops would ultimately serve in the Korean War, only about 40,000 troops from the other UN Peace-Keeping nations were involved, and of these, half were British. In fact, the tiny non-American units actually tended to get in the way and confuse American planning more than they actually helped the war effort. Chiang Kai-Shek offered 35,000 Chinese Nationalist troops, but Truman and Acheson rejected this, afraid the involvement of Chinese Nationalists might provoke the involvement of the much larger Red Army of the PRC. Clearly, the UN forces were an instrument of US policy designed to give the appearance of international consensus rather than a truly autonomous international organization.
MacArthur's call for American ground troops was based on several factors. First of all, bad weather was limiting the accuracy of air power to defend the South Koreans. Second, the ROK troops could not now be given tanks and be expected to use them: only well trained US soldiers could operate the tanks and anti-tank weapons necessary to halt the advance of the Soviet T-34s operated by North Korean fighters.
Although Truman did not immediately give MacArthur all the troops he wanted, once some ground troops were committed to Korea, it was inevitable that more would follow. Once committed in this battle against Communism, the US could not afford to lose for fear of losing its credibility with all of its allies, especially the NATO powers. The same scenario would play out a decade later in Vietnam. In this way, like a brush fire, "Limited Wars" in the modern-era often proved (and prove) very difficult to contain, to keep "limited."
The American troops MacArthur brought in from Japan had not seen fighting in years, if ever. An occupation army, most of these troops were under-trained and out of shape. As he held on to Pusan, MacArthur's forces became more and more fit. The intense heat of summer was also a problem, and American morale was extremely low during this period. The worst was seen in "bug-out fever", where US troops would flee battle, throwing down weapons as they ran. The North Korean troops were battle-hardened veterans by comparison, used to the terrain, formidable fighters, and highly mobile. Once more, in the comparison of North Korean quick-strike capabilities and firm resolve versus an under-motivated and slow to react American army, this early phase of the Korean War foreshadowed the Vietnam War. Also, as in Vietnam, strategic bombing, which had played such a vital part in World War II, never worked very well against North Korea, which simply wasn't industrialized enough for bombing to have a devastating impact.