Iran has virtually completed a “Shiite Crescent” of influence across the heart of the Middle East, using a string of battlefield successes to link a network of allies and proxy forces now spanning from nation’s border with Iraq all the way to Lebanon.

The crescent, a longtime strategic goal meant to confront rival Sunni Arab powers led by Saudi Arabia, includes several semipermanent bases established in territory claimed as the radical Salafi Sunni Islamic State movement recedes. It also presents a strategic challenge to the U.S. presence in the region that the Trump administration is still struggling to meet.

Pentagon officials say they have military options to challenge Iran’s growing regional clout, but senior U.S. commanders have little appetite for direct confrontation — particularly if it has a high likelihood of drawing Iranian proxy forces in Iraq, Syria and Lebanon.

With Islamic State fighters now squeezed into small pockets of territory in Syria’s southern Deir el-Zour region and elsewhere along the Euphrates River Valley, the U.S.-led coalition battling the terrorist group hopes to begin paring back its military operations in the region. This summer, by contrast, Iranian-allied militias set up a road link stretching from Iran’s western border to Syria’s Mediterranean coast.

Iran is making its greatest progress to date toward its goal of creating a land connection from Iran to Lebanon, where Tehran-backed Hezbollah militants have a long-established political foothold, said Sarhang Hamasaeed, the head of Middle East Programs at the U.S. Institute of Peace.

“It may not be the desired path [Iran] wanted, but they now have a path” for an uninterrupted land bridge lined with allied or proxy forces directly from Tehran to the Mediterranean Sea, Mr. Hamasaeed said in an interview on Monday.

Tehran has long sought the land bridge to Lebanon, where the Shiite militant movement Hezbollah has proved to be its most potent ally in the rivalry with Sunni states and with Israel, with which Hezbollah has repeatedly clashed.

The physical link to Lebanon “is a symbolic win for the Iranians,” said Jennifer Cafarella, the senior intelligence planner at the Washington-based Institute for the Study of War.

While the Islamic Revolutionary Guard Corps, or IRGC, has long had a sophisticated logistics operation in place to move weapons, materiel and supplies to Lebanese Hezbollah as well as other proxy forces in Iraq and Syria, the emerging Shiite Crescent is a testament to Tehran’s ability enlist proxy forces among indigenous regional forces to challenge its adversaries.

“What you [now] have, nevertheless, is a slick operation in Tehran that is looking to project its influence far beyond its borders — and it has been quite successful in doing this with relatively little effort,” said H.A. Hellyer, a senior nonresident fellow on Middle East affairs at the Washington-based Atlantic Council. “It’s been a lot of return on very little investment.”

That return on investment has been substantial.

In Iraq and Syria, Iranian-backed Shiite militias known as the Popular Mobilization Forces, trained and supported by Iranian military advisers from the Islamic Revolutionary Guard Corps and other Iranian government entities, have become an increasingly powerful influence in territory once held by Islamic State.

“The PMF is the guarantor” of the land bridge crossing Iraq, Mr. Hamasaeed said.

Tehran is also increasing pressure on the government of Iraqi Prime Minister Haider al-Abadi to fully incorporate the Popular Mobilization Forces into the country’s national security apparatus.

Prominent Iranian Shiite cleric Ali al-Husseini al-Sistani on Friday pressed the Baghdad government to fully implement plans to fold the Shiite militias into the country’s armed forces. Mr. al-Abadi formally federalized the militias as the campaign to drive Islamic State from its northern Iraqi stronghold of Mosul kicked off last year.

In Syria, Iranian paramilitaries constitute the bulk of President Bashar Assad’s ground forces, which have been slowly gaining ground in the wake of Islamic State’s defeats in the country.

Regime forces, backed by Russian airstrikes, retook the Syrian rebel stronghold of Aleppo last year. Since then, government troops and Iranian militias have quickly secured former Islamic State redoubts in al-Bab, Deir el-Zour and other strongholds in the Euphrates River Valley.

“It is an expeditionary force,” Ms. Cafarella said about the array of paramilitary and proxy forces in Iraq and Syria that are allied under the Iranian flag. “It is much more than just Lebanese Hezbollah.”

Building the bridge

Much of the construction of the Crescent took place in the shadows, “below the threshold” that might have sparked a stronger U.S. response, Ms. Cafarella said. While Washington and its allies focused on the war against Islamic State, Tehran was positioning itself “to dominate the peace after ISIS,” she said.

That U.S. focus on Islamic State “afforded Iran so much time and freedom of action” to organize their proxy forces that the country’s land bridge into Lebanon is now a reality, Ms. Cafarella said.

The Iranian exile opposition group Mujahedeen-e-Khalq (MEK), which fiercely opposes the Islamic regime in Tehran, says Iran’s presence in Syria goes far beyond what most regional analysts say. A recent report by the MEK-tied National Council of Resistance of Iran, which in the past has produced major exposes of Iran’s nuclear and military establishment, concluded that the “Iranian regime had deployed 70,000 fighters to Syria” by July 2016.

It also said Tehran has spent $100 billion on the Syrian civil war over the past five years and pays $1 billion in salaries for Islamic Revolutionary Guard Corps and Shiite paramilitary mercenaries deployed there.

But growing opposition from Sunni forces inside Iraq could fracture the Shiite militias’ hold on territory, said Mr. Hamasaeed. “It is not a done deal yet,” he said.

Iraqi lawmakers are pressing the al-Abadi government to rein in the Shiite paramilitaries. They argue that the benefits of having the militants on the battlefield are outweighed by the influence their presence affords Tehran inside the country.

Salim al-Jabouri, speaker of the Iraqi parliament and a Sunni Muslim, last month called on Mr. al-Abadi to disband the majority of the Shiite paramilitary groups over growing fears of Iranian influence sparking renewed sectarian violence in the country.

“We need to bring balance to the Iraqi military” and larger security forces, he said last month on a visit to Washington, noting that not all of the Shiite paramilitaries organized underneath the banner of the Popular Mobilization Forces posed a threat to the country’s stability.

Iraqi Vice President Osama al-Nujaifi, the highest-ranking Sunni in the al-Abadi government, said the Shiite Popular Mobilization Forces “have their own political aspirations, their own [political] agendas. … They are very dangerous to the future of Iraq.”

But Tehran could look to Iraq’s parliamentary elections next year to politically solidify the Popular Mobilization Forces’ battlefield gains in Iraq, Mr. Hamasaeed said.

The political wings of the Shiite-led Badr Organization, Asa’ib Ahl al-Haq (also known as the Khazali Network), and Kata’ib Hezbollah are already rallying support among Iraq’s Shiites to lock in parliamentary seats in next year’s elections. The armed factions of these groups have been accused of sectarian violence and extrajudicial killings during operations against Islamic State in Fallujah.

Mr. Abadi could curb the rising political clout of the Shiite groups by fully integrating the Popular Mobilization Forces into the Iraqi security forces, but that move has perils of its own. If Baghdad is unable to bring the Popular Mobilization Forces to heel, then the move could cement Iran’s influence into the Iraqi political system for decades, Mr. Hamasaeed said. “We are now at a point of competition on which of these two scenarios could happen,” he added.

Limited options

There is little indication that the Trump administration is planning a major push to curb those efforts. On the campaign trail and in the Oval Office, Mr. Trump has largely kept the focus on the fight to eradicate Islamic State.

Members of Mr. Trump’s national security team signaled this year that rolling back Tehran’s proxy armies in the Middle East would be a main pillar of the administration’s get-tough policy with Tehran, but few details have emerged.

President Trump has focused much of his attention on the nuclear issue and the 2015 deal that the Obama administration helped negotiate. Mr. Trump has also moved closer to Israel and cultivated better ties with Sunni powers such as Saudi Arabia and Egypt in an apparent bid to curb Tehran’s regional ambitions.

But with Islamic State’s grip on the Middle East now effectively shattered and Iran’s goal of forging a land bridge into Lebanon now within grasp, the attention of Washington and the Arab world has begun to shift, Mr. Hellyer said.

“As ISIS goes on the back burner in terms of priorities, Iran is going to be far more of a first-level priority” for the U.S. and Arab allies such as Saudi Arabia, he said.

But there are limits to how much the U.S. administration — and the U.S. public — want to get involved in the Middle East’s long-standing rivalries, Mr. Hamasaeed said.

“The U.S. military does not have the appetite to engage this militarily,” he said.

The U.S. has sought to apply pressure away from the battlefields of the region. U.S. officials elevated their public condemnation of Hezbollah on Tuesday, adding that two of the Iran-backed terrorist group’s top operatives to a special State Department most-wanted list and asserting that all of the group’s factions — even those holding political office in Lebanon — are part of the same terrorist operation.

Washington has provided Lebanon with more than $1.5 billion in military assistance since 2006, and U.S. special operations forces have been providing training and support for the Lebanese army since 2011, according to Reuters.

More recently, CIA Director Mike Pompeo attempted to reach out to Islamic Revolutionary Guard Force chief Maj. Gen. Qassem Soleimani, and other top Iranian security officials, warning Tehran to rein in its efforts to expand its influence in Iraq and Syria.

“What we were communicating … was that we will hold [Mr. Soleimani] and Iran accountable for any attacks on American interests in Iraq by forces that are under their control,” Mr. Pompeo said Sunday during the annual Reagan National Defense Forum in Southern California.

But short of military action, there is no incentive for Iran’s hard-line leaders to acquiesce to Washington’s demands and scuttle years of effort to forge a land bridge to Lebanon, Ms. Cafarella said.

“Iran has no intent or interest in surrendering any of its gains in Iraq and Syria,” she said.

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