Why were Ambassador Chris Stevens and three other Americans murdered by Libyan jihadists? The preliminary round of official investigations may focus on decisions by mid-level officials at the State Department that deprived the Benghazi mission of adequate security, and a failure by the large CIA team in the city to detect the imminent threat from extremist groups.

But ultimately the disaster in Libya derived from Obama’s doctrine. Having been reluctantly dragged by France and Britain into intervening in Libya’s revolution, Obama withdrew U.S. planes from the fight as quickly as possible; when the war ended, the White House insisted that no U.S. forces stay behind. Requests by Libya’s fragile transition government for NATO’s security assistance were answered with an ill-conceived and ultimately failed program to train a few people in Jordan.

A new report by the Rand Corporation concludes that “this lighter-footprint approach has made Libya a test case for a new post-Iraq and Afghanistan model of nation-building.” But the result is that, a year after the death of dictator Moammar Gaddafi, Libya is policed by what amounts to a mess of militias. Its newly elected government has little authority over most of the country’s armed men — much less the capacity to take on the jidhadist forces gathering in and around Benghazi.