Well let's not put ALL the blame on Mac:
Oct 12 1950 - CIA intelligence analysis asserts that Communist Chinese intervention in the Korean War is "not probable".
Oct 27 1950 - 300,000 Communist Chinese troops invade Korea.
NICE work.
McCarthur did get conflicting intelligence reports.
But having won his most brilliant victory of Inchon, he got overconfident and began to see what he wanted to see, a fatal flaw in any general.
A ROK unit had been decimated by Chinese troops and Marines had fought a large number of them as they got close to the Chinese border.
McCarthur was urging the 1st Marine divison to move faster, but the Marine commander smelt an ambush and held back, saving the division from annihilation.