American intelligence failed to detect the 180,000 Chinese troops sneaking across the Yalu because the Chinese traveled at night and hid in the mountains during the day. US aerial reconnaissance proved unable to detect these troop movements.
MacArthur, still hoping to one day run for President, became obsessed with an absolute victory. He began trying to manipulate Washington, massaging information so that Washington would order him to do exactly what he wanted. Was MacArthur's behavior simply based on an enormous egotism? Some have suggested that he was manic-depressive, and suffered mood swings. Infuriated by MacArthur's actions, Truman certainly wanted to remove him from command, but MacArthur still remained too popular in the US.
Many believed that PRC intervention in the Korean War might herald the coming of World War III. Though such a prospect seemed possible, Truman could not simply allow a US retreat, because that would leave South Korea and Syngman Rhee in the lurch, ruining American credibility with regard to its other alliances. Truman had wanted to fight a "limited war," but once engaged, found that circumstances of war in the modern world did not allow for the concept of limited war. The world powers, in their ideological struggle, had their interests everywhere.
many Republican Senators and Congressman continued to support MacArthur and the idea of a full-scale strike against the PRC. But despite his threat to use the A-Bomb, Truman probably did not mean it. The US was still the only country that had ever dropped an A-Bomb, and dropping one again, on yet another Asian nation, might seem unduly aggressive and racist.