The Korean war was a war between South Korea and North Korea. South korea had the support of the United Nations and United States of America, while North Korea was principally supported by the Soviet union and China.
General MacArthur wanted the expansive use of America's military endowment to fight the war but President Truman objected. It will be greatly unwise at that time to go by MacArthur's advocacy because great countries like the USSR possessed as many and powerful ammunition as the United States. It would be difficult for the United States to go to war with the USSR and China at that point and come out victorious.
This is evident by the short term reception MacArthur received from the people when he was relinquished of his position. The people turned down the overwhelming support they initially gave to him as soon as they realized this.
Moreover President Eisenhower that succeeded Truman won this war without necessarily going into it with full force as suggested by MacArthur.