1950’s mess was a UN mission
Now we seem to be alone.
It was that, although the United States and the ROK did most of the heavy lifting and General MacArthur was the Supreme Commander -- although he spent most of his time in Tokyo enjoying the comforts he had as the "Emperor of Japan" and rarely visited Korea more than for a few hours.
It became, at least cosmetically, a UN mission because Russia -- in pursuit of its efforts to have mainland China seated in place of Taiwan -- was boycotting the UN and hence did not veto the Security Council resolution requesting help in Korea from member states.
General MacArthur, like all the rest of us, was flawed. At seventy and the most senior of all serving U.S. officers of flag rank, he often ignored the "advice" coming from his juniors on the Joint Chiefs of Staff as well as occasionally from President Truman. He referred to General Eisenhower as the "best clerk I ever had." He did seem to think highly of General Ridgeway, who ultimately replaced him.
However, General MacArthur was in many respects a military genius as his Inchon invasion demonstrated. He also recognized the threats of Mainland China and Russia. He was largely responsible for the absence of Russia from Japan as he managed its very successful rehabilitation -- compare the position of Russia in Europe at the same time.