For the first time since the ouster of Col. Muammar el-Qaddafi in 2011, the independent militias that dominated Libya’s biggest cities and sometimes cowed the central government have fled from the streets, chased away by a combination of civilian protesters and armed groups. But instead of a triumph for the transitional government of Prime Minister Ali Zeidan, the retreat has marked a new stage in Libya’s descent into chaos. In Tripoli, the capital, the government is now struggling to fill the vacuum left by the sudden disappearance of the militias, which had controlled scores of government facilities and private properties. In Benghazi, it has been unable to slow an escalating campaign of assassinations and bombings that are believed to be the work of extremist militiamen who have gone underground; now the attacks are targeting the unit that passes for the government’s only security force. And where […]