World Bulletin/News Desk

Whether Washington likes it or not, Shi'ite Iran, a key ally of the governments of Iraq and Syria, is all but certain to be a major player in the fight against the Sunni militants of ISIL.

Bitterly at odds over most major Middle Eastern issues, the United States and Iran now essentially find themselves on the same side in an escalating crisis over the group that has seized large swathes of Iraq and Syria.

While hardly anyone sees this as a case of "the enemy of my enemy is my friend," an old proverb that resonates even today through the geopolitics of the region, a common foe could give Washington and Tehran rare common ground.

Any effort to bring Iran and the United States closer will not be easy.

Outright U.S.-Iranian military cooperation against ISIL appears to be off the table and Iran is also being frozen out of membership in the international coalition being assembled by President Barack Obama.

Current and former U.S. officials acknowledge that American and Iranian interests rarely coincide, so the challenge will be to make sure Iran's role is as constructive as possible.

"They will clearly be part of this," said Aaron David Miller, a former Middle East adviser to Republican and Democratic administrations. "They have boots on the ground in ways that we don't . But there's good reason for us to be wary."

Washington's preferred dynamic, experts say, is for Tehran to work separately toward the goal of defeating ISIL while the two countries seek to "deconflict" their activities, essentially to avoid stepping on each other's toes.

The United States cut off diplomatic ties with Tehran during a hostage crisis after the 1979 Islamic revolution, and the Obama administration has made curbing Iran's disputed nuclear program a high priority.

"Their irresponsible behavior reminds us that the enemy of our enemy may still remain our enemy," the former head of the U.S. military's Central Command, retired General James Mattis, told a congressional hearing last week. "There may be ways that we can work in parallel. But I'd be very cautious and have very modest expectations."

Any material cooperation between the United States and Iran, for example in sharing intelligence about ISIL movements, would have to be behind the scenes or done through intermediaries, experts say.

The reason is that Washington's Sunni Arab partners in the fight against ISIL regard Iran with even more suspicion than U.S. officials and see it as trying to deepen Shi'ite dominance in Iraq and extend its own influence in the region.

WASHINGTON'S DILEMMA

Iran's backing for allied Iraqi Shi'ite militias poses a particular dilemma for Washington. U.S. officials are mindful that the groups have helped blunt ISIL's momentum in Iraq after the Iraqi army collapsed in the north in the face of ISIL's onslaught.

But Washington also sees the potential for further alienation of the large Sunni minority that has helped fuel the rise of ISIL.

"What the U.S. wants is (for Iran) to create some measure of discipline over these militias' activities so that it doesn't become widespread attacks against Sunnis," Miller said.

It would not be the first time the United States and Iran have found common cause and become uneasy bedfellows.

During the U.S.-led invasion of Afghanistan in late 2001, officials from both sides say Iranian intelligence cooperation was invaluable in routing the Taliban and securing the advance of the Northern Alliance rebel forces. Information from Tehran helped to target air strikes and win over tribal groups.

Some U.S. and other Western officials privately say that working with Tehran in a similar way might make sense in the fight against ISIL, but they concede that geopolitical rivalries and sectarian hostilities between Iran and Sunni coalition members will make it impossible to coordinate this time.

At the same time, experts say there is relatively little the United States can do to stop Iran from weighing into the fight against ISIL on its own terms as Tehran continues efforts to prop up the government of Syrian President Bashar al-Assad and strengthen its clout with Iraq's Shi'ite dominated leadership.

"The U.S. cannot stop Iran from following its own agenda when it comes to Iraq and Syria," says Hayat Alvi, professor of Middle Eastern studies at the U.S. Naval War College, adding that distrust between the two nations remained enormous. "That makes for a very poor partnership."

Even so, Tehran has emerged as a ready source of weaponry for Baghdad, provided much more quickly and with fewer strings attached than U.S. military aid, and Washington appears to have quietly acquiesced to this arrangement.

A detachment of Su-25 Russian-built attack jets delivered at short notice in early July were almost certainly supplied to Iraq by Tehran, London's International Institute for Strategic Studies said after examining assorted photographs.

With the launching overnight of the first U.S.-led air strikes against ISIL targets in Syria, Washington may be counting on Tehran to ensure that Assad keeps his air-defense system in check.

The campaign against ISIL offers further prospects for the United States and Iran to take advantage of a tentative thaw in relations that started last year with Obama's diplomatic outreach to Tehran, which resulted in the latest rounds of nuclear talks with Western powers.

But some Washington-based analysts caution against placing too much trust in Iran's intentions or its willingness to help ease the ISIL crisis.

"If the Iranians really wanted to do something substantive, they would have done it long ago," said James Carafano, a military analyst at the conservative Heritage Foundation think tank. "A better U.S. strategy would be focusing on minimizing Iranian influence."

BRITISH PM TO MEET IRAN'S ROUHANI TO ASK FOR HELP

Meanwhile, British Prime Minister David Cameron will meet Iranian President Hassan Rouhani in the next two days in New York and ask him for help to fight ISIL, the first meeting between leaders of the two nations since Tehran's 1979 Islamic revolution.

Cameron will meet Rouhani on the sidelines of the United Nations General Assembly, a source in the British leader's office told Reuters, speaking after the United States and Arab partners struck ISIL targets inside Syria.

Cameron is in New York to try to bolster international action and to clarify his own country's position when it comes to air strikes, something London has so far held off participating in.

Cameron is expected to ask Rouhani to drop his support for Syrian President Bashar al-Assad, to tell him that developing nuclear weapons is unacceptable, and to urge him to join the international coalition.

Britain said in June it would reopen its embassy in Tehran after a hiatus of more than two and a half years after relations between the two countries began to thaw after last year's election of Rouhani, a man London considers a relative moderate.

Cameron is also expected to hold talks with U.S. President Barack Obama in New York about the possibility of Britain joining air strikes, something he has signalled he would need the approval of the British parliament to do.