Kim Jong-il’ death and the succession of Kim Jong-un has fueled increased concern about the possibility of government collapse in North Korea. Many observers wonder if Kim will be able to successfully consolidate power in Pyongyang. People worry that his failure to secure power would lead to a power struggle or an implosion, and possibly to regime collapse. To be sure, such a collapse has been predicted for decades, and the regime many “uddle through”for many years to come. Nevertheless, it is vital to plan ahead for the contingency of collapse because of the magnitude of potential problems, the complexity of mitigating them, and the serious dangers associated with mismanaging this effort. Coordinated planning, particularly between the United States, the Republic of Korea (ROK), and China, is essential to avoid massive loss of life among North Koreans and dangerous escalation between countries that might step in to stabilize the country after collapse. Government collapse in Pyongyang could create anarchy or a vacuum in which several serious problems might develop on the peninsula. These might include the disappearance of “loose nukes”and other weapons of mass destruction (WMD) across international borders; a humanitarian disaster, if government-provided food and health services cease in the chaos of collapse; and the potential for ongoing insurgency and violence. Hunger and insecurity could trigger a massive refugee crisis if North Koreans take flight in search of food and safety. Many people assume that under such circumstances, international relief agencies would step in to provide humanitarian assistance. But convoys of food and medicine cannot be sent into anarchic areas without military escort-particularly into a country like North Korea, which has a vast security apparatus (military, internal security, reserves) that could disrupt relief efforts. Fearing refugees and seeking to secure WMD, various countries might choose to perform military missions to stabilize North Korea following a government collapse. Such missions may include (1) stability operations to secure roads, provide public security, and deliver humanitarian relief; (2) finding, securing, and eliminating North Korea’ WMD program; (3) border control; (4) the disarmament of North Korean military forces, in order to disarm potential insurgents; and (5) a rapid reaction force standing by to deter or subdue any insurgent or military activity that might interfere with the provision of aid. In this study we describe these missions and calculate their requirements. We did not discuss whose troops should perform these missions; rather, we calculated their rough force requirements in a generic sense. For our analysis we made the assumption that collapse (which could occur in more or less dangerous ways) occurred in a more benign manner. We made this assumption not because we thought it was the most plausible, but because it was the most analytically useful. Our study shows that even in a best-case collapse scenario, the requirements for stabilizing the peninsula are staggering, and the potential for miscalculation and escalation significant. In a more dangerous collapse scenario, missions would be more numerous and more difficult, and thus their requirements higher. For our analysis, we drew upon a variety of metrics, historical and theoretical, to estimate force requirements for the various missions. For example, analysts of stability operations typically estimate force size based on the size of the population to be pacified. Historically, easier operations (with little to no popular resistance) have required about 4 peacekeepers per 1,000 people in the population; harder operations have required closer to 20 per 1,000. For our study, we adopted a mid-range estimate of 13 peacekeepers per 1,000 people, which (given the North Korean population of 23 million) yields a requirement of 312,000 peacekeepers just for the stability operation—ontrolling roads and other transportation infrastructure; transporting, protecting, and distributing food and medicine; and taking over the responsibilities of the police to provide public security. Mobilizing and inserting a force of this size would confront significant logistical and diplomatic challenges, making a simultaneous operation (in which over 300,000 peacekeepers stabilized the entire country at once) improbable. Thus we model a sequenced approach in which North Korea is administratively divided into horizontal tiers. Stability forces would advance upward into the first tier from South Korea, stabilize that tier, and subsequently advance upward into the next tier. Because of the danger of anarchy in Pyongyang and the northern part of the country while the stability force works its way northward, this effort should be complemented by a second element in which stability forces move into North Korea from two of its major ports (Nampo and Chongjin), establish control over the lines of communication, and conduct stability operations. Staffing all five missions, under this relatively benign scenario, would require 300,000 to 400,000 personnel. Table 1 (below) summarizes our calculations. As discussed above, many analysts would object to our assumption of a relatively benign collapse scenario. They might argue that North Korea’ military will fight to the bitter end, or that regional warlordism is likely to develop among elites. Our analysis has important implications for analysts who make these more pessimistic (and quite plausible) assumptions: stabilizing North Korea under more dangerous circumstances would require tens of thousands more troops than we calculate here. Because of the size and complexity of these missions, and because of the perils associated with mismanaging them, this analysis suggests that advance and combined planning is essential. Combined planning should include those actors (e.g., China, South Korea, and the United States) who would otherwise likely take highly destabilizing unilateral action to protect their own interests. Furthermore, this analysis has important implications for South Korean defense planning: it suggests the need to reform the way reserves are called up, and it warns that significant cuts in South Korean ground force would risk Seoul’ ability to stabilize a collapsed North Korea.