Iraqi forces prepare to fire mortars at Kurdish peshmerga positions near the area of Fishkhabour, located on the Turkish and Syrian borders in the Iraqi Kurdish autonomous region. (Ahmad Al-Rubaye/AFP/Getty Images)

President Trump's assertive new strategy toward Iran is already colliding with the reality of Tehran's vastly expanded influence in the Middle East as a result of the Islamic State war.

The launch of the strategy signaled an important shift in U.S. Middle East policy away from an ­almost exclusive focus on fighting the Islamic State to an effort that also pushes back against years of Iranian expansion in the region.

But the strategy offers no specifics for how to confront Iran's pervasive presence on the ground in Iraq, Syria and beyond, raising questions about how easy it will be to push back against Iranian influence without triggering new conflicts.

The difficulty of changing tack on what has amounted in recent years to a tacit alliance with Iran in Iraq for the purpose of fighting the Islamic State was evident this week in a sharp ­rebuke by Iraqi Prime Minister Haider al-Abadi to Secretary of State Rex Tillerson's call for ­Iranian-backed Iraqi militias to "go home."

The militias are "part of the Iraqi institutions," Abadi responded in a statement. "The Hashd al-Shaabi should be encouraged because they are the hope of the country and the region," he said, referring to the coalition of militias known as the Popular Mobilization Forces.

One of Iraq's most powerful militia leaders went further, saying in a tweet that it is the Americans who should go home.

"Your armed forces have to prepare from this point immediately and without any delay to leave our homeland Iraq after the end of the excuse of the ISIS presence," said Qais al-Khazali, who heads the Iran-backed Asaib Ahl al-Haq militia and was imprisoned for two years by U.S. forces for his role in organizing an attack that killed five U.S. soldiers in 2006. ISIS is another name for the Islamic State.

The rising tensions risk a return to the era of proxy wars that prevailed in the middle of the last decade, when Iranian-backed militias blew up American troops and U.S. and Iranian allies kidnapped one another's operatives on the streets of Baghdad — or even in the 1980s, when Americans were driven out of Beirut by suicide bombings and the kidnapping of dozens of Western hostages by a group allied to the Iranian-backed Hezbollah movement.

The tensions also risk complicating the final stages of the war against the Islamic State, which has now been mostly confined to a desert stretch of highly strategic territory spanning the border between Syria and Iraq.

[After Raqqa, the U.S. sees Russia, Assad looming over remaining Syrian battlefield]

In the three years since the United States intervened in both countries to roll back the Islamic State, Iran has dispatched tens of thousands of allied militia fighters to the battlefield, asserting its presence in vast swaths of territory not previously regarded as within Iran's traditional orbit of influence.

Iranian-backed Shiite militias have fanned out into the Sunni and Christian areas of northern and western Iraq that were freed from Islamic State control — often with the help of U.S. airstrikes. Now these forces are aiding efforts by the Iraqi government to reclaim territory that has been controlled by U.S.-allied Kurds since as long ago as 1991. Alongside the Iraqi army, the militias are moving toward the small Kurdish-controlled border crossing of Fishkhabour, through which the U.S. military sends its military support to Kurds engaged in the fight against the Islamic State in Syria. A change in control of the crossing could jeopardize the ability to supply U.S. troops and allies for the fight in Syria, where they are competing with Iran and its allies for control of the last significant pocket of territory controlled by the militants.

Iran has already asserted itself as a major player in Syria by supplying, arming and training the tens of thousands of militia fighters that have proved instrumental in securing President Bashar al-Assad's hold on power.

Alongside Assad's army, they are now in a race with the U.S.-backed Syrian Democratic Forces to capture the last stretch of Islamic State territory in eastern Syria. There they hope to link up with their Iraqi allies to secure a land bridge between Tehran and their Lebanese ally Hezbollah in Beirut, securing an arc of continuous Iranian influence that stretches to the Mediterranean.

Tillerson on Thursday disputed suggestions that Syria has fallen under Iranian influence, telling reporters in Geneva that Russia is responsible for most of the government's gains. "I do not see Syria as a triumph for Iran. I see Iran as a hanger-on. Iran has not particularly been successful in liberating areas," he said.

In all of their maneuvers, the militias are operating with the blessing of the sovereign governments, making it hard to discern where Iran's agency begins and the governments' ends.

The relationships are, how­ever, part of a deliberate strategy by Iran to wield influence through local proxies that operate in cooperation with governments, said Mohammed Obeid, a Beirut-based political analyst who is close to Hezbollah. Much in the way that the Iranian-backed Hezbollah has woven itself into the fabric of the Lebanese state, Iran's allies in Syria and Iraq are establishing themselves as vital but parallel components of the national security forces, he said.

"The Iranians were smart, and they still are smart. They wanted to export their revolution. But they found that they don't need to overthrow the governments in the region," Obeid said. "They can create political and military groups alongside these governments, and Hezbollah is the prime example. Hezbollah is now part of the government, part of parliament, and it is a strong military force on the ground. Which no other force in the world can destroy.

"They are succeeding in what they wanted to do, which was to spread their influence throughout the region."

Iran's frequent boasts about the influence it commands are questioned by many analysts. The Hashd al-Shaabi in Iraq are not monolithic and include some groups answerable to the government and to the moderate Shiite clergy, said Maria Fantappie of the International Crisis Group. The Islamic State war has also helped legitimize Abadi, and Iraqis themselves are fiercely ­nationalistic, she said.

In Lebanon, Hezbollah has to balance the interests of its Sunni and Christian partners in government. In Syria, Iran has to contend with a population that contains few adherents of the branch of Shiite Islam represented by the Iranian leadership, giving it no natural constituency in the country.

Tehran is nonetheless in a strong position to push back against any U.S. efforts to curb its reach, said Jennifer Cafarella of the Institute for the Study of War.

"The bottom line is that the U.S. strategy against ISIS empowered Iran in the region by accepting Iran as a de facto partner in the fight against ISIS. And American leverage has been decreasing proportionate to the territory taken away from ISIS," she said.

Iran may feel it has secured enough regional clout to decide against risking its gains by confronting the United States, Cafarella added. But should Tehran feel threatened by the Trump administration's hostile rhetoric, it has plenty of tools at its disposal to resist, she said.

They include the loyalty of the same militia groups that hounded U.S. troops with lethally effective roadside bombs and mortar bombardments in the years immediately before the U.S. withdrawal from Iraq in 2011 and took Western hostages at different times in both Iraq and Lebanon.

"If somebody does that to you, you have to retaliate or leave. If you retaliate, you've got an escalation ladder and you have to go in and fight. And if you leave, you put yourself in a worse situation," said Faysal Itani of the Washington-based Atlantic Council.

The U.S. military maintains about 5,200 troops in Iraq, according to Abadi, and 500 Special Operations troops in Syria, according to the Pentagon. They are heavily outnumbered by the militias and would be vulnerable to a revival of the Shiite insurgency that was responsible for a majority of U.S. casualties in the final years of the Iraq War.

Any attempt to challenge Iran in Iraq and Syria "will have a very negative effect on the existence of American forces in the region," said Obeid. "They are surrounded by Iranians. This is a big question the Americans are going to have to face."

For now, U.S. officials seem to be pinning their hopes on a new round of sanctions against Hezbollah in Lebanon and Iran's Revolutionary Guard Corps, which commands Tehran's network of allied militias.

But sanctions alone won't dent Iran's geographical reach, and the Trump administration appears to have no appetite for a full-scale confrontation, Itani said.

Whether such a confrontation would work in America's favor is also in doubt, said Nicholas Heras of the Center for a New American Security. Leaders in both Iraq and Syria are aware that Iran will remain its neighbor throughout any shifts in U.S. policy.

"No amount of strategizing and boots on the ground short of an invasion is going to push back against Iran's ability to shape events in Baghdad and all of Iraq," he said. "And short of the U.S. Army marching on Damascus, Iran will not be ripped from Syria or Lebanon."

Karen DeYoung in Washington and Tamer El-Ghobashy in Baghdad contributed to this report.

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