From the outside, it seems almost inexplicable that Mr. Stevens was in Benghazi on the anniversary of 9/11, in the midst of increasing attacks on foreign targets. Some of his colleagues in Tripoli say it was just bad timing, that Mr. Stevens felt this was the only time period available for an essential trip, and that every day in Libya was dangerous. But none of that explains why the compound was lightly guarded, why the United States was relying on fickle Libyan militias for defensive support, and why extra security resources were sent back to the States shortly before the attack.

Still, the deeper question is why the United States and its NATO allies believed that international responsibilities to Libya would end with military action, and that Libya would somehow right itself. We will probably never get to have a meaningful discussion about this, as long as we are tantalized by theories about conspiracies or political malfeasance.

When power shifted from the rebel capital of Benghazi back to Tripoli in late 2011 with the ouster of Colonel Qaddafi, fear began taking hold in the eastern provinces. The Islamists began appropriating public assets like hospitals. They had already assassinated a former Qaddafi interior minister who had become a rebel commander. Nevertheless, exemplary local elections were held in May 2012, and before the ambassador’s death Benghazi seemed, at least on the surface, to be far more orderly than Tripoli, with its warring militias and criminal gangs.

But now, Ambassador Stevens’s death has led to the undermining of an already weak central authority, the further isolation of Benghazi from Libya’s political center, a slowing of American and international technical aid and the empowerment of the Islamists.