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Hussein Malla/Associated Press

An explosion last Thursday rocked a Hezbollah stronghold in Beirut, one day after authorities announced the arrest of a senior Saudi-born Qaeda leader.

BEIRUT, Lebanon — The images of recent days have an eerie familiarity, as if the horrors of the past decade were being played back: masked gunmen recapturing the Iraqi cities of Falluja and Ramadi, where so many American soldiers died fighting them. Car bombs exploding amid the elegance of downtown Beirut. The charnel house of Syria's worsening civil war.

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 both Iraq and Syria under the banner of Al Qaeda, as the two countries' conflicts amplify each other and foster ever-deeper radicalism. Behind much of it is the bitter rivalry of two great oil powers, Iran and Saudi Arabia, whose rulers — claiming to represent Shiite and Sunni Islam, respectively — cynically deploy a sectarian agenda that makes almost any sort of accommodation a heresy.

"I think we are witnessing a turning point, and it could be one of the worst in all our history," said Elias Khoury, a Lebanese novelist and critic who lived through his own country's 15-year civil war. "The West is not there, and we are in the hands of two regional powers, the Saudis and Iranians, each of which is fanatical in its own way. I don't see how they can reach any entente, any rational solution."

The drumbeat of violence in recent weeks threatens to bring back the worst of the Iraqi civil war that the United States touched off with an invasion and then spent billions of dollars and thousands of soldiers' lives to overcome.

With the possible withdrawal of American forces in Afghanistan looming later this year, many fear that an insurgency will unravel that country, too, leaving another American nation-building effort in ashes.

The Obama administration defends its record of engagement in the region, pointing to its efforts to resolve the Iranian nuclear crisis and the Palestinian dispute, but acknowledges that there are limits. "It's not in America's interests to have troops in the middle of every conflict in the Middle East, or to be permanently involved in open-ended wars in the Middle East," Benjamin J. Rhodes, a White House deputy national security adviser, said in an email on Saturday.

For the first time since the American troop withdrawal of 2011, fighters from a Qaeda affiliate have recaptured Iraqi territory. In the past few days they have seized parts of the two biggest cities in Anbar Province, where the government, which the fighters revile as a tool of Shiite Iran, struggles to maintain a semblance of authority.

Lebanon has seen two deadly car bombs, including one that killed a senior political figure and American ally.

In Syria, the tempo of violence has increased, with hundreds of civilians killed by bombs dropped indiscriminately on houses and markets.

Linking all this mayhem is an increasingly naked appeal to the atavistic loyalties of clan and sect. Foreign powers' imposing agendas on the region, and the police-state tactics of Arab despots, had never allowed communities to work out their long-simmering enmities. But these divides, largely benign during times of peace, have grown steadily more toxic since the Iranian revolution of 1979. The events of recent years have accelerated the trend, as foreign invasions and the recent round of Arab uprisings left the state weak, borders blurred, and people resorting to older loyalties for safety.

Arab leaders are moving more aggressively to fill the vacuum left by the United States and other Western powers as they line up by sect and perceived interest. The Saudi government's pledge last week of $3 billion to the Lebanese Army is a strikingly bold bid to reassert influence in a country where Iran has long played a dominant proxy role through Hezbollah, the Shiite movement it finances and arms.

That Saudi pledge came just after the assassination of Mohamad B. Chatah, a prominent political figure allied with the Saudis, in a downtown car bombing that is widely believed to have been the work of the Syrian government or its Iranian or Lebanese allies, who are all fighting on the same side in the civil war.

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.