But for all its echoes, the bloodshed that has engulfed Iraq, Lebanon and Syria in the past two weeks exposes something new and destabilizing: the emergence of a post-American Middle East in which no broker has the power, or the will, to contain the region’s sectarian hatreds. Amid this vacuum, fanatical Islamists have flourished in Iraq and Syria under the banner of Al Qaeda, as the two countries’ conflicts amplify each other and foster ever-deeper radicalism.