The hugely expensive US attempt at nation-building in Afghanistan has had only limited success and may not survive an American withdrawal, according to a two-year congressional investigation. The report calls on the administration to urgently rethink its assistance programs as President Barack Obama prepares to begin drawing down the number of US troops in Afghanistan this year. The report, prepared by the Senate foreign relations committee's Democratic majority staff, comes as Congress and the American public have grown increasingly restless about the human and economic cost of the decade-long war and reflects growing concerns about Mr Obama's war strategy even within his party.