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of general war.” But on November 24 as MacArthur lunged toward the Yalu River border, theCIA still found insufficient evidence to suggest a Chinese plan for “major offensive operations.” Intelligence agencies did not lack information; instead the problem resided at the level of assumptions and presuppositions: Moscow wouldn’t intervene because it would fear global war; Beijing wouldn’t either, because Moscow dictated to its leaders.
    The Russians and the Chinese had a division of labor before the war started: Russian military advisers were in North Korea and Chinese military advisers were in North Vietnam in 1950. Both worked with the respective armies on strategic planning, logistics, army organization, and political controls. While the Koreans prepared their invasion, the Vietnamese “were planning a full-scale assault on the French forces along the Sino-Vietnamese border.” 23 This was less a conscious or planned division of labor than a result of Soviet occupation of North Korea and Chinese occupation of northern Vietnam after World War II, and connections between Mao and Ho Chi Minh during the Yanan period.
    A Chinese military intelligence group arrived in Pyongyang within three weeks of the war’s start, and as early as August 4 Mao considered intervening in Korea: if the Americans were to invade the North “we must therefore come to [North] Korea’s aid and intervene in the name of a volunteer army.” Around the time of the Inchon landing a high North Korean officer, Pak Il-yu, requested Chinese military assistance, and then on October 1 Kim Il Sung held an emergency meeting with the Chinese ambassador to plead that the PLA 13th Army Corps quickly cross the Yalu River. By then Chinese intervention was certain, the only question was the timing: on September 30 Mao told Stalin “we have decided” to send as many as twelve infantry divisions. The Kremlin, however, fretted that a big Chinese offensive against the Americans might precipitate a world war, and backed off from a previous commitment to provide airpower to protect China’s coasts. China went ahead regardless, which apparently surprised Stalin. 24
    North Korean and Chinese documents make clear that Chinadid not enter the war purely as a defensive measure to protect its border, as has long been known, but also because Mao determined early in the war that should the North Koreans falter, China had an obligation to come to their aid because of the sacrifice of so many Koreans in the Chinese revolution, the anti-Japanese resistance, and the Chinese civil war. The PRC’s Foreign Affairs Ministry referred to China’s obligations to “the Korean people who have stood on our side during the past decades.” The October 1 crossing of the 38th parallel caused Mao a sleepless night, but he made the lone decision to intervene, and informed Stalin of his decision the next day. As if some telepathy were at work, MacArthur told the Department of the Army on the same day that “the field of our military operations is limited only by military exigencies and the international boundaries of Korea. The so-called 38th Parallel, accordingly, is not a factor.” 25 In other words, NSC 81, the rollback strategy itself, caused the Chinese intervention, and not the subsequent arrival of American troops at the Yalu River.
    Chinese forces attacked in late October, bloodied many American troops, and then disappeared. It is likely that the Chinese hoped this would suffice to stop the American march to the Yalu, perhaps at the narrow neck of the peninsula above Pyongyang. But this also would leave the DPRK as a small, rump regime. Around this time Kim Il Sung arrived in Beijing on an armored train, moving under cover of darkness and blanketed security. He was accompanied by three other uniformed Koreans, and China’s northeast leader, Kao Kang. High PRC leaders, including Chou En-lai and Nieh Jung-chen (the two besides Mao most closely linked to the Korean decision),