Suspicion in last week's attack that killed the U.S. ambassador to Libya and three other Americans has focused on members of a hardcore Islamist militia known for its sympathies to al-Qaida, its fierce animosity to the U.S. and its intimidation of other Muslims who don't conform to its harsh ideology. That doesn't mean Libyan authorities will move against Ansar al-Shariah soon. The group is among the most powerful of the many, heavily armed militias that the government relies on to keep security in Benghazi. In fact, it guards one of Benghazi's main hospitals. Libya's militias are a legacy of last...