Off By That Much
"At headquarters, the agency kept advising Truman that China would not enter the [Korean] war on any significant scale. On October 18, as MacArthur's troops surged north toward the Yalu River and the Chinese border, the CIA reported that 'The Soviet Korean venture has ended in failure.' On October 20, the CIA said that Chinese forces detected at the Yalu were there to protect hydro-electric power plants. On October 28, it told the White House that those Chinese troops were scattered volunteers. On October 30, after American troops had been attacked, taking heavy casualties, the CIA reaffirmed that a major Chinese intervention was unlikely. A few days later, Chinese-speaking CIA officers interrogated several prisoners taken during the encounter and determined that they were Mao's soldiers. Yet CIA headquarters asserted one last time that China would not invade in force. Two days later 300,000 Chinese troops struck with an attack so brutal that it nearly pushed the Americans into the sea."
—from Tim Weiner's "Legacy of Ashes: The History of the CIA," pp. 58-59, beginning, or continuing, a tradition of faulty intelligence that invariably missed the biggest foreign policy events of the 20th century and beyond.