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How does one define collapse? The loss of central authority in the capital of Pyongyang, while fast-moving in a historical sense, might nevertheless play out over weeks rather than days. So at what point does China move forces across the Yalu and Tumen rivers to prevent a massive flight of refugees northward? At what point does the South Korean military act? Because North Korea is a heavily militarized state, the direction in which its commanders decide to defect -- to Beijing or to Seoul -- will be critical.