https://youtu.be/HoKe2QvBXZk
McArthur vs Truman

U.S. failed in Intelligence twice during Korean War. 

U.S. did not think that North Korea would invade South Korea.

Consequently, U.S. pulled out the entire U.S. troops from

South Korea in June 29, 1949 leaving extremely ill equiped South Korean

military alone a year before the war. 

McArthur denied the Chinese army will intervene if U.S. troops

 cross 38 parallel and ignored the intelligence reports that Chinese 

were indeed in NK. He also underestimated the strength of 

Chinese army.