Iranian leaders will have an opportunity to cement their status as the world's leading state sponsor of terrorism once a U.S.-led coalition drives the Islamic State from their last remaining strongholds, according to Republican and Democratic lawmakers.

"The day after Raqqa falls is going to be the moment that Iran moves to try to oust the United States from the region," Sen. Marco Rubio, R-Fla., told the Washington Examiner.

That wouldn't leave much time to celebrate the capture of the Islamic State's capital city. An Iranian offensive would represent more than one terrorist threat succeeding another, because they're collaborating with a Russian government that aspires openly to create a "post-West world order." Lawmakers are concerned the destruction of ISIS as a territorial power could inaugurate a new phase in the regional rivalry with the United States.

"I do think there will be some collusion between Russia and Iran," New York Rep. Eliot Engel, the top Democrat on the House Foreign Affairs Committee, told the Washington Examiner. "I worry about it."

Russo-Iranian domination of Syria could have cascading negative consequences for U.S. allies and eventually Americans. Syria shares a border with Israel, Iraq and Turkey — a NATO ally. That makes it an important theater in Russian efforts to replace "the so-called 'new liberal order'" with a new paradigm that shifts power away from the United States.

"If you want, you can call it a "post-West" world order," Russian Foreign Minister Sergey Lavrov said Saturday at the Munich Security Conference.

Iran, the world's leading Shia Muslim power, is a key factor in that Russian effort. They are natural rivals of Saudi Arabia and other Sunni Muslim governments that work with the United States. And because Iran-backed Shia militants have a history of committing atrocities against Sunnis, they strengthen radical terrorist Sunni groups such as ISIS and al Qaeda.

"These groups intentionally exploit such conditions by portraying themselves as the only reliable defenders of the Sunni Arabs in both countries," according to a report from the Institute for the Study of War.

It was also just a year ago that Iran detained and humiliated 10 U.S. sailors at gunpoint in the Persian Gulf when they entered the nation's territorial waters. Explosives supplied by Iran caused nearly 1,000 U.S. troops in Iraq between 2005 and 2011. So American policymakers are tasked with defeating ISIS in a way that prevents another terrorist group from taking its place, without yielding the region to Russia and Iran.

"You're not saying oh, there's going to be an attack on you in Michigan tomorrow; it's that the world becomes less safe and the United States position and that of it's allies becomes less safe if the Iranians and the Russians own Syria," said Tony Badran, a research fellow at the Foundation for Defense of Democracies. "All of the sudden a rogue, terrorist-sponsoring regime on the cusp of nuclear power says, 'hey, we own this place,' not you and starts taking shots at U.S. sailors, and U.S. ships, and U.S. allies, and all the sudden you have a breakdown of order."

There's no appetite for a large-scale invasion of Iraq or Syria to defeat ISIS, so the U.S. has tried to cultivate local fighting forces who could do the job with the help of a relatively small number of American troops. That effort has been complicated by the tangle of ethnic and sectarian rivalries in the region. The enemy of the enemy is also the enemy — or, alternatively, is also the enemy of our friends, to revise the old proverb.

That's true in Iraq, where several thousand U.S. troops are coordinating a coalition that includes the Kurds (an ethnic minority in northern Iraq), the Iraq government forces and Shia militants. Many of those militias were organized by the Iranian military before then-President Barack Obama intervened to stop ISIS. Iran has plenty of friends in the Iraqi parliament, where the largest single bloc of lawmakers are Shia. And those lawmakers are already citing Trump's executive order that sought to temporarily suspending travel from Iraq as an excuse to break with America.

"Why should we trust the new American administration?" an ally of Maliki, the former pro-Iran prime minister, said after the order was released. "We have the right to get closer to Iran as a secure ally in order to preserve our national interests."

Iraqi Prime Minister Haider al-Abadi invokes the threat of ISIS to justify ongoing cooperation with the United States, but that argument might not be as effective after ISIS is expelled from Mosul. "[Iran will] immediately begin the push to take away all of our influence and try to push us out of there, and probably use their Shia allies to threaten our troops and other personnel on the ground," Rubio predicted.

It's even more complicated in Syria, where Obama deployed several hundred U.S. special forces operators to advise a coalition fighting against ISIS. That's a controversial group of fighters, however. The Arab forces have fought alongside al Qaeda against Syrian President Bashar Assad, as the Russians have observed to justify bombing U.S.-backed groups. And the Syrian Kurds are affiliated with a Kurdish terrorist group in a long-running conflict with Turkey, so cooperating with them has strained the U.S.-Turkey relationship.

"They're a very capable fighting force, but they're also considered a terrorist organization by a member of NATO," Rubio said. "So if they try to unify the cantons across northern Syria, you're going to have a Turkish military incursion that will create real complications."

Russian President Vladimir Putin has been developing a closer relationship with Turkish leaders, so lawmakers worry that he might join the attack in order to help Assad and reap the benefits of helping a NATO ally destroy U.S.-backed forces.

"I think it's something that we have to take into account and that's why you need tough talk to Iran and to Russia to make them understand this is not something that can be taken lightly," Engel said. "And there is question as to whether the Trump administration is capable of talking to tough to Russia because there seems to be so many ties to Russia that you wonder if indeed they can talk tough to them."

A Turkish military incursion against ISIS would be preferable, but Sen. Lindsey Graham thinks they're unwilling to do so unless they're fighting alongside 8,000 to 10,0000 U.S. troops.

"The moderate forces have been really hit hard by Assad and Russia," the South Carolina Republican told the Washington Examiner. "So, what I would recommend the administration think about doing is replacing the Kurds, not completely, but the Turks are willing to go in on the ground, they're will to help us liberate Raqqa."

Even some lawmakers worried about Iranian and Russian influence might not support such plans, however. "You don't want to make the same mistake we did in Iraq and get involved in a ground war," Engel told the Washington Examiner.

Defense Secretary Jim Mattis has been tasked with developing a comprehensive strategy to defeat ISIS, while also staking out a much harder line with respect to Russia than Trump has done personally. The anti-ISIS plan he develops will be a test of his ability to navigate domestic American politics, contain the ethnic and religious rivalries of Syria, and blunt Russia's grand strategic aims in the region.

"It's not easy, there's no easy solution," Engel said. "We have to just monitor it. I just know two things: Iran is not our friend and Russia's not our friend."