The United Nations did not only help USA by providing troops from the eighteen states but also by providing different kind of support (including to replace the obsolete weapons of USA). Therefore with the help of the UN, USA had accomplished its very first aim even faster. Thus the war can be ended earlier.
Furthermore, with the help of the USSR and China, the North Korean was able to defend the USA and the UN troops to reach a stalemate situation. Therefore continuing the war would only cause the loss of mass of civilian and soldiers. The war already caused the death of 5000,000 South Korean civilians, 30, 000 American soldiers, 70, 000 South Korean soldiers and 4500 United Nations soldiers. The number of death was already so astonishing and furthermore the economic cost was not included. Thus the war should not be continued any further.
General Macarthur should not be allowed to attack the North Korea because USA must also consider the consequence of the diplomatic relationship between China and USSR. Also, it would be clearly an act of invasion to invade North Korea further. Although the relationship of USA and USSR was already not so friendly, the Korean War might worsen it into bitter enemies of war. Furthermore, China had a profound influence in Asia. Having a bad diplomatic relationship with China, USA might have great difficulties and problems in its foreign policy on Asia.
Thus General Macarthur must be removed for his aggressive policy would only result to a larger scale of war than the Korean War (with the involvement of USSR and China). He even suggested using nuclear weapons which will virtually destroy the whole world. Furthermore, he blatantly ignored the