Just hours before he died in a terrorist attack at the U.S. compound in Benghazi, Ambassador Chris Stevens sent a cable to Secretary of State Hillary Clinton painting a chaotic, violent portrait of the eastern Libya city and warning that local militias were threatening to pull the security they afforded U.S. officials.

Militia leaders told U.S. officials just two days before the attack that they were angered by U.S. support of a particular candidate for Libyan prime minister and…