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The death toll in Syria has reached a threshold where outside parties are entitled to use force to protect the civilian population. But those who would intervene in ostensible pursuit of this noble objective have an obligation to ensure that, on balance, the patient benefits from the procedure. Whereas former president Bill Clinton can claim with some justification that "the burning of villages and killing of innocents was history" after NATO's 1999 intervention in Kosovo, achieving a positive balance sheet in Syria will be extraordinarily difficult.