FOR many Americans the killing of Christopher Stevens, their ambassador to Libya, this week crystallised everything they have come to expect from the Arab world. In a country where the West only last year helped depose a murderous tyrant, a Salafist mob attacked the American consulate in Benghazi, killing Mr Stevens and three colleagues. The trigger for this murder, the riots in neighbouring Egypt and the storming of the American embassy in Yemen? A tacky amateur video about the Prophet Muhammad that the Obama administration had already condemned. Why on earth, many Americans are asking, should the United States try to police a region, when all it gets in return is mindless abuse, blame for things it cannot control, and mob violence?