Iraqi police stand guard during patrol at Rabia, near the main border between Iraq and Syria, on March 2, 2013. Clashes between the Syrian army and rebels has brought the civil war close to neighbouring Iraq. (REUTERS)

Syria’s civil war is increasingly threatening to destabilize neighboring Iraq, widening a sectarian divide in a nation still reeling from the messy aftermath of the U.S.-led invasion a decade ago.

Iraqi officials have expressed alarm in recent weeks as fighting between forces loyal to President Bashar al-Assad and the armed opposition has spilled across the border. After staying on the sidelines for more than a year, Sunni tribes in Iraq that straddle the frontier have decisively joined the effort to topple the Alawite Shiite-led government in Damascus.

Many officials here fear that a growing Iraqi Sunni protest movement that has found inspiration from the uprising next door could quickly turn into all-out revolt in regions that formed the heart of the Sunni insurgency over the past decade.

“We will be the most affected if violence spreads in a way that cannot be controlled,” Ali al-Moussawi, a spokesman for Iraqi Prime Minister Nouri al-Maliki, said in an interview. “What worries us is that there is no plan to control things in Syria, to find a peaceful political solution or a type of change that can be controlled.”

The Iraqi government is wedged between Iran, a Shiite Islamic republic that has provided crucial aid to Assad, and the Sunni countries that support the Syrian opposition, which include Turkey and Persian Gulf states.

Caught in the middle: The contrast in lives between those who stayed back and those who decided to leave Syria.

Maliki, though a Shiite often aligned with Iran, is no fan of Syria’s embattled president. Maliki believes Assad helped fuel Iraq’s sectarian war by permitting weapons and fighters to stream across the border in the past decade. But analysts and officials say Iraq’s premier has shown a growing willingness to turn a blind eye to Iran’s aid for Assad because he feels increasingly threatened by the prospect of a rebel victory in Damascus.

“This is drawing the gulf states back into this part of the Middle East in a way they haven’t been since the Gulf War, and it’s drawing them in a way that is deeply sectarian,” said Tamara Cofman Wittes, director of the Saban Center for Middle East Policy at the Brookings Institution in Washington and a former senior State Department official. “For Iraq’s Sunnis, this is an opportunity to get patronage or support, but it’s in a context of increasingly fierce sectarian polarization in the Arab world, and that’s quite serious.”

Iraq’s minority Sunnis took to the streets nationwide late last year, using Arab Spring-like tactics to protest what they consider their political marginalization after the fall of Saddam Hussein, a fellow Sunni. Since then, as rebels have made inroads across Syria, Iraq’s Sunnis have become increasingly resolute, Iraqi officials and Western diplomats say.

“Many people are emboldened, as they think this is their chance,” said a senior U.S. diplomat in Baghdad, speaking on the condition of anonymity to discuss a delicate policy issue. “There is increased pressure on Baghdad. They [the Sunnis] tell themselves, ‘The tide is flowing our way.’ ”

Remnants of Iraq’s once-
potent Sunni insurgency are probably vying for a comeback, the diplomat added.

“The terrorists, I have no doubt, see this as their moment of opportunity and are trying to capitalize on it,” he said.

Cross-border jihadist links

Jabhat al-Nusra, a hard-line Islamist group that has engineered many of the rebels’ decisive tactical victories in Syria, is closely linked to the Sunni insurgent group al-Qaeda in Iraq. In online forums used by jihadists, members have posted statements vowing to take on Maliki after they drive out Assad and proclaiming that they will march on Baghdad after seizing Damascus.

The convergence of militant groups in both countries became starkly clear this month when Iraqi insurgents ambushed a convoy transporting Syrian soldiers who had fled into Iraq after a firefight along the northern border. The attack occurred near Rabiyah, an Iraqi village and a stronghold of the large Shammar tribe.

The tribe’s leader, Abdullah al-Yawar, said his fellow tribesmen in Syria joined the rebellion against Assad about six months ago and have since sought to oust government forces from their areas.

“They became involved because they determined the Syrian revolution needs to be for all people,” he said in an interview.

The attack near Rabiyah, which killed 48 Syrian soldiers and nine Iraqi guards, underscored the ease with which insurgents operate along the porous border.

“If Syria develops into a civil war, which would lead to partition on a sectarian basis, that would lay the groundwork for al-Qaeda and other groups that don’t recognize borders,” Falah al-Fayyadh, Maliki’s national security adviser, said in a recent interview, referring to the type of conflict that might unfold if Assad is ousted. “The real danger for Iraq is if al-Qaeda is able to impose its influence on part of Syria.”

U.S. cooperation, frustration

Iraqi officials have asked Washington to expedite the sale of heavy weapons that could be used to crack down on insurgents along the border. Moussawi, the government spokesman, said the weapons are needed to “help us prevent any kinds of violations, like establishing terrorist camps” on the Iraqi side of the border.

That narrow policy goal has led to collaboration between U.S. intelligence personnel and the Iraqi government, according to officials from both sides. But Washington and Baghdad remain divided over Iraq’s broader approach to the Syrian conflict.

U.S. officials have chided Baghdad for allowing Iran to ferry weapons to Syrian troops over Iraqi airspace.

On a trip here last weekend, Secretary of State John F. Kerry lobbied Maliki to stop allowing Iranian arms and fighters to cross into Syria through Iraq, saying this was helping to bolster Assad’s government. Maliki denied aiding the Syrian government.

Robert Ford, the U.S. ambassador to Syria, warned of the same danger in congressional testimony last week. “That cannot continue without consequences,” he said.

Ford also cited reports that Iraqi Shiite militias linked to political factions have been dispatched to the Syrian battlefield in support of Assad’s forces.

“We want the Iraqi government to understand that it has no interest in having an extremist government in Syria,” the ambassador said. “And the longer the conflict continues, the greater the influence of extremists on the ground.”

Karen DeYoung in Washington contributed to this report.