The killings of the US ambassador to Libya and three other American diplomats in an attack on the US consulate in Benghazi Tuesday underscore how anti-authoritarian revolutions across the Arab world have unleashed extremist Islamist forces violently opposed to America and its ideals. “The big question for the United States, and it’s only been made more urgent by these events, is how to adjust to a world where [moderate Muslim] governments have paved the way for more extremist elements to wield their influence,” says Patrick Clawson, director of research at the Washington Institute for Near East Policy in Washington. “It’s...