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Can the United States win the war in Afghanistan? The antiwar left has long held the war is unwinnable. Now some conservatives are arguing that President Obama's weakness and indecision forecast American failure--and that, if we're going to fail, we should just get out now.

We would be the last to defend Obama's indefensible dithering. But the war in Afghanistan remains both winnable and worth winning--even with Obama as president. And no form of withdrawal or defeat is consistent with safeguarding key American interests in a volatile and dangerous region of the world.

President Obama's apparent reluctance to pursue the fight does not inspire confidence. But he did send General Stanley McChrystal to take command, along with 21,000 additional troops. Despite efforts by...