MacArthur's intelligence officer, General Willoughby, chose to tell MacArthur exactly what he wanted to hear: that the Chinese threat to us in Korea was minimal. Keiser bore the brunt of that mistake, as his troops were the first to be attacked en masse by overwhelming CHICOM forces, and were routed with very heavy casualties.
MacArthur, despite many warnings, used Willoughby's flawed intelligence to devise an extremely foolhardy strategy, which divided his army into two units that were unable to support each other as they were separated by many miles of high mountains, placing overall command in the hands of General Salmon, who screwed up even further. The JCS had immediately seen the military error of MacArthur's plans, but were mesmerized and awed by MacArthur and did not object. It was a disaster, and only the US Marines prevented it from becoming a massacre. The mistake ended MacArthur's career and soured his legacy.
MacArthur wanted nuclear intervention in China, The Truman Administration and the UN did not. Our troops and the UN contingent were battered for months until General Ridgeway assumed command ... but in the end, all that was achieved was a bitter stalemate and equally bitter recrimination that continues to this day about MacArthur. IMNVHO, MacArthur should be honored for his accomplishments, but not forgiven for his last military blunders in Korea.
And is it just coincidence that reversal of US fortunes in war fighting coincides with our involvement in the UN?