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…

Iran and Saudi Arabia have increased their efforts to arm and recruit fighters in the civil war in Syria, which top officials in both countries portray as an existential struggle. Sunni Muslims from Egypt, Libya, Tunisia, Saudi Arabia and elsewhere have joined the rebels, many fighting alongside affiliates of Al Qaeda. And Shiites from Bahrain, Lebanon, Yemen and even Africa are fighting with pro-government militias, fearing that a defeat for Bashar al-Assad, Syria’s president, would endanger their Shiite brethren everywhere…

Although the Saudi government waged a bitter struggle with Al Qaeda on its own soil a decade ago, the kingdom now supports Islamist rebels in Syria who often fight alongside Qaeda groups like the Nusra Front. The Saudis say they have little choice: having lobbied unsuccessfully for a decisive American intervention in Syria, they believe they must now back whoever can help them defeat Mr. Assad’s forces and his Iranian allies.