The killing of U.S. Ambassador Christopher Stevens in Benghazi, Libya, on the anniversary of September 11 should provoke a similar examination of the Obama presidency. Here was a quintessential ambassador – an Arabic speaker, popular among ordinary Libyans, who had worked with the revolutionaries to overthrow Muammar Gaddafi – killed in the line of duty.  It was a colossal failure, marking a nadir in a presidency that boasted a break with Bush-era unilateralism and a promise to “commit ourselves to an effort – a sustained effort – to find common ground” with the Muslim world.