Syria's protracted civil war is spilling across its borders, creating breeding grounds for extremists, sharpening sectarian schisms and threatening to destabilize U.S. allies in the Middle East.
The war has attracted jihadists from across the region, including Libya, where rebels overthrew Moammar Gadhafi's regime a year ago and where al Qaeda in the Islamic Maghreb has sought to put down roots.
"If al Qaeda-related groups gain a foothold in Syria, that is very bad news for everybody," said Danielle Pletka, vice president of foreign and defense policy studies at the American Enterprise Institute.
"And if governments that have long been allies of the U.S. – [I'm] thinking here of a country like Jordan – end up being destabilized, that is also potentially very harmful for the United States," she said. "There are so many wild cards."
In just the past month, a mortar shell fired by the Syrian military killed five civilians in Turkey, provoking a Turkish attack on Syrian targets; a top Lebanese intelligence official was assassinated in Beirut by a car bomb blamed on Syria; and a Jordanian soldier was killed in a border clash with armed men trying to cross over from Syria.
"Fallout from the Syrian civil war is one of the most important strategic issues facing the United States today," said Daniel L. Byman, deputy director of the Saban Center for Middle East Policy at the Brookings Institution.
"The conflict has the potential to destabilize key regional states, like Iraq, and to assist terrorist groups, like Hezbollah and al Qaeda. It also implicates the interests of powerful friends like Israel and Turkey," Mr. Byman said. "As the conflict intensifies, these problems are likely to grow worse, not better."
Syrian President Bashar Assad's regime agreed Thursday to a four-day cease-fire effective from Friday to mark the Muslim holiday of Eid-al-Adha. The Syrian army, however, reserved the right to respond to rebel attacks during this period.
Rebels reported heavy fighting in the western cities of Al Rastan and Aleppo in interviews over Skype on Thursday.
Syrian activists say more than 33,000 people have been killed since the start of the uprising against the Assad regime in March of last year. The United Nations estimates that more than 20,000 people have died.
Foment in Lebanon
The civil war has exacerbated sectarian tensions in Syria's neighborhood. The consequences have been felt most violently in Lebanon, its southwestern neighbor, where Sunnis are pitted against Mr. Assad's Shiite supporters. Mr. Assad belongs to the Alawite branch of Shiite Islam.
Lebanon's sectarian rivalries were brought to the fore Oct. 19, when Brig. Gen. Wissam al-Hassan, a top intelligence official and foe of Syria, was killed by a massive car bomb in Beirut.
The blast laid bare the tenuous nature of peace in Lebanon, which was ravaged by a civil war between 1975 and 1990 and occupied by Syrian forces until 2005.
Al-Hassan had led an investigation that had resulted in the arrest in August of Michel Samaha, a former Lebanese Cabinet minister and ally of Mr. Assad's. Mr. Samaha has been accused of smuggling weapons from Syria into Lebanon with the aim of fomenting violence. Two Syrian officers also have been indicted in that plot.
Lebanese politicians say al-Hassan's opposition to Syria cost him his life.
"I think the Syrians are trying to tell to the world: 'If the Syrian regime is toppled, or if the Syrian regime is in danger, the whole region will be in danger,'" said Boutros Harb, a leader in the March 14 Alliance, a coalition of Lebanese political parties and independent candidates that opposes the Assad regime.
"The developments in Syria are so dangerous [they] cannot happen without having any consequences on the Lebanese situation," Mr. Harb said Monday at the Aspen Institute in Washington, D.C.
"What's happening in Syria is a period of change. This change, unfortunately, is not helping things," he added.
Hisham Melhem, a Lebanese journalist and Washington bureau chief of Al-Arabiya, a Dubai-based news channel, said Gen. al-Hassan's assassination "is another reminder that it is naive, maybe dangerously naive, to believe that Lebanon can 'disassociate' itself from the storm that is engulfing Syria."
"From the beginning, everybody who knew anything about Lebanon, Syria would have told policymakers in Washington that unless the uprising succeeds or [is] defeated quickly, three things are inevitable: that the regime will make violence worse; that the regime will use sectarianism, and that sectarianism will draw the jihadists; and it is inevitable that the conflict in Syria, if it is allowed to drag on for months, will spill over to the neighboring countries," said Mr. Melhem, who also spoke at the Aspen Institute.
The sectarian issue has been a factor in the response of Syria's neighbors to the crisis.
"One of the reasons why [Iraqi Prime Minister Nouri] al-Maliki is not supporting the rebels in Syria, despite the fact that he has no affection for Assad, is because he is really worried about western Iraq and the implications a substantial presence of Sunni extremists will have in that part of Iraq," said Ms. Pletka. Mr. al-Maliki is Shiite.
Earlier this month, the conflict threatened to draw in Turkey when a Syrian shell landed on a Turkish village, killing five civilians. Turkey's parliament authorized a military response, and its military bombed targets inside Syria.
The incident prompted a frantic scramble to calm tensions by officials, including Secretary of State Hillary Rodham Clinton, U.N. Secretary-General Ban Ki-moon and Lakhdar Brahimi, the U.N. and Arab League's envoy to Syria. Turkey is a member of NATO, and under the military alliance's Washington Treaty, an attack against one member is considered an attack against all members.
The Turkish newspaper Milliyet reported over the weekend that Turkey's military had, in recent weeks, fired 87 times on Syria in retaliation for Syrian shells landing inside Turkey. Twelve Syrian soldiers had been killed and several tanks destroyed, it said.
In June, Syrian forces shot down a Turkish military jet, killing two pilots.
Syria's crisis could have long-lasting repercussions in the region. Besides Lebanon, Jordan also risks being destabilized.
Jordan's King Abdullah II warned of the "dangerous repercussions" of the conflict on the region.
"There can certainly be permanent damage and lasting effect of the Syrian conflict in these other countries even after the conflict in Syria is resolved," said Michele Dunne, director of the Atlantic Council's Rafik Hariri Center for the Middle East.
Rebels continued to fight regime forces late Thursday.
In an apparent setback for the regime, activists said rebel fighters pushed into predominantly Christian and Kurdish neighborhoods in northern Aleppo that previously had been held by pro-Assad forces, the Associated Press reported.
"It was a surprise," local activist Abu Raed said via Skype. "It was fast progress and in an unexpected direction."
He asked to be identified only by his nickname for fear of reprisals.
The battle for Aleppo, a former regime stronghold and Syria's business hub, has been largely deadlocked since rebels first captured parts of the city in late July.
A complete rebel takeover could change the momentum of the war, although in recent months, front lines have shifted repeatedly and it was not clear if rebel fighters could maintain Thursday's gains, the AP reported.
Activists also reported fighting and shelling by government forces near the capital of Damascus, and scores of people were reported killed nationwide.
The civil war has created more than a quarter of a million refugees who have poured across Syria's borders into Turkey, Jordan, Lebanon and Iraq. Those refugees have heightened security concerns and, with the onset of winter, are straining precious resources.
The influx of refugees "has been particularly destabilizing in Lebanon, but I think there are worrying signs in Jordan and Turkey as well," Ms. Dunne said.
"All of this is, unfortunately, very much to be expected and will get worse until the conflict inside Syria is resolved in some way," she added.
International efforts to resolve the crisis have made little headway. Russia and China have vetoed three U.N. Security Council resolutions that sought to find a solution to the conflict.
President Obama's Republican challenger, Mitt Romney, has advocated a more robust leadership role for the U.S. in efforts to resolve the crisis. While Mr. Romney is opposed to U.S. military intervention in Syria, he supports arming the rebels.
Some Arab states already are arming the rebels. The Obama administration has been reluctant to do so and has provided only nonlethal aid.
The Syrian crisis has "potential upsides" in terms of the damage it can do to Iran, Syria's ally in the region, and its ability to help Syria rejoin the community of nations, Ms. Pletka said.
"The problem is, the longer it spirals out of control, the more likely it is that there are going to be genuine downsides," she added.
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