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An unquiet ghost stalks the White House Situation Room as Barack Obama, increasingly Hamlet-like, ponders what to do in Afghanistan: it is the spectre of the Vietnam War, America’s enduring historical hang-up.

Comparisons between these two conflicts are easy to make, but hard to avoid: a grinding, unpredictable battle in difficult terrain, a weak and corrupted foreign government kept afloat by American guns and money; a versatile enemy, adept at ambush warfare, with sanctuary in a neighbouring country.

American public opinion on Afghanistan is shifting in a way reminiscent of the tide of feeling that brought the Vietnam War to its humiliating close. The death toll is ramping up, with 55 US servicemen killed this month — the war is suddenly being brought home...