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Mr Obama should learn from Mali as he gives effect to his agreement with Hamid Karzai for an accelerated withdrawal of US forces from Afghanistan. Mr Obama's enthusiasm to bring his troops home quickly is understandable - his nation has paid a heavy price - but the withdrawal must be measured rather than hasty and he should heed the Pentagon's advice and leave at least 20,000 troops beyond 2014 to deal with any new threat. The suggestion that this could be whittled down to 3000, or even a "zero option", leaving the inexperienced Afghan forces to keep order, is alarming. Mali shows the speed with which the jihadists exploit a power vacuum.