"In order to confront Iran or push back more fiercely against it, you may find you're in a conflict far more far-reaching and more destructive to the global economy than many of our allies or American public are willing to bear," said Nicholas Heras of the Centre for a New American Security.
Iran's alleged quest to produce a nuclear weapon - which Tehran has always denied - has been curbed by the nuclear accord signed in 2015. But in the meantime it has developed missiles capable of hitting US bases and allies across the Middle East and a network of alliances that have turned it into the most powerful regional player.
Iran now stands at the apex of an arc of influence stretching from Tehran to the Mediterranean, from the borders of Nato to the borders of Israel and along the southern tip of the Arabian Peninsula.
It commands the loyalties of tens of thousands of allied militias and proxy armies that are fighting on the front lines in Syria, Iraq and Yemen with armoured vehicles, tanks and heavy weapons.
They have been joined by thousands of members of the Iranian Revolutionary Guard Corps, Iran's most prestigious military wing, who have acquired meaningful battlefield experience in the process.
For the first time in its history, the Institute for the Study of War noted in a report last week, Iran has developed the capacity to project conventional military force for hundreds of kilometres beyond its borders.
"This capability, which very few states in the world have, will fundamentally alter the strategic calculus and balance of power within the Middle East," the Institute said.
America's Sunni Arab allies, who blame the Obama Administration's hesitancy for Iran's expanded powers, are relishing the prospect of a more confrontational US approach.
Any misgivings they may have had about Trump's anti-Muslim rhetoric have been dwarfed by their enthusiasm for an American president they believe will push back against Iran.
"We are so happy and excited about President Trump," said Abdullah al-Shammari, a former Saudi Arabian diplomat, speaking from the Saudi capital of Riyadh. "We expect him to deal with the Iranians as the threat that they are, producing missiles and interfering in other countries."
Exactly what the Trump Administration intends to do about a state of affairs that has already become deeply entrenched is unclear, however.
So pervasive is Iran's presence across the region that it is hard to see how any US Administration could easily roll it back without destabilising allies, endangering Americans, undermining the war against Isis (Islamic State) and upsetting the new regional balance that emerged during the Obama Administration's retreat, analysts say.
The Trump Administration has given no indication that it intends to abrogate the nuclear accord. Rather, US officials say, the goal is to contain activities that lie outside the scope of the accord, such as the ballistic missile programme and what one official called the "destabilising activities" of the Iranian Revolutionary Guard Corps and its proxies.
So far, US action has been confined to retaliation for Iran's test-fire of a ballistic missile last week and an attack by Yemen's Houthi rebels on a Saudi Arabian navy ship in the Red Sea. The Treasury imposed sanctions last week against people and companies alleged to be involved in the missile programme and the Pentagon dispatched the naval destroyer USS Cole to the coast of Yemen, suggesting that Iran's arming of the Houthis may be an early target.
Otherwise, the Trump Administration has given little indication of what it has in mind, except to make it clear that it intends to be different from Obama.
"Iran is playing with fire - they don't appreciate how 'kind' President Obama was to them. Not me!" Trump wrote in a tweet at the weekend.
Iran has offered a relatively muted response to the challenge, with Iran's Foreign Minister tweeting that Iran is "unmoved" by the threats emanating from Washington. "We'll never initiate war," he said.
Iran may well conclude that it is not in its interests to engage in confrontation with a new US Administration already earning a reputation for unpredictability, analysts say.
But those familiar with Iran's behaviour in the region have said that they do not believe it will readily surrender its gains.
"Any pushing back, the Iranians won't take it lying down," predicted Mowaffak al-Rubaie, a Shia Iraqi parliamentarian who has, for many years, worked to bridge the divide between Iran and America in Iraq.
"Iraq, Iran and the United States are an extremely finely balanced equation, and Trump shouldn't come and bash," he said. "He should play this extremely delicately."
IRAQ
It is in Iraq, where fighting Isis has most conspicuously brought the United States into a tacit alliance with Iran, that a more hostile relationship between Tehran and Washington could prove most consequential.
Iranian-backed militias are deeply embedded in the overall Iraqi effort to wrest back territory from the militants, one that is also being aided by the United States. In the Mosul offensive, hundreds of US advisers are working alongside Iraqi troops advancing from the east, among about 6000 US troops currently deployed in Iraq. Thousands of Iranian-backed militias are meanwhile advancing on the city from the west, among a force of tens of thousands that answers mostly, though not exclusively, to Iran.
One of the Iranian-backed groups fighting around Mosul is Kataib Hizbollah, which also blew up American troops with roadside bombs and fired mortars into US bases at the height of US-Iranian tensions a decade ago. It will not hesitate to attack US troops should the US attempt to diminish Iran's role in Iraq, said Jaffar Al-Hussaini, Kataib Hizbollah's spokesman.
"We look at America as our first enemy, the source of all evil on the Earth," he said. "American interests in Iraq are within our sights and our fire range. If they act foolishly, their interests will be wiped out . . . and we can target their bases whenever we want."
SYRIA
It is also hard to see how the US could act to curtail the extensive influence acquired by Iran during the war in Syria. Iran and Russia together have fought to ensure the survival of President Bashar al-Assad's regime, and they are now pursuing a peace settlement in alliance with Turkey that excludes a role for America. The US has been left with few friends and little leverage, apart from the Kurds in the northeast of the country.
Russia controls the skies over Syria, and Turkey wields influence over the rebels, but Iran holds sway on the ground, through its extensive network of Shia militias drawn from Lebanon, Iraq, Afghanistan and Pakistan. They have provided the manpower for front lines from the northern countryside of Aleppo, near the Turkish border, to the Golan Heights bordering Israel in the south.
Trump's promises to curb Iranian influence are at odds with his stated desire to pursue closer co-operation with Russia in Syria and also to support Assad, because Iran is allied with both Assad and Russia, said Mustafa Alani, a director at the Dubai-based Gulf Research Centre.
"He will not be able to contain Iran if he is going to support Assad. He cannot have both at the same time," he said. The solution, he said, is to topple Assad, because "Assad is the man who is underpinned by Iranian support. He was saved only by Iranian intervention."
Alani sees no reason Trump should not easily be able to contain Iranian influence.
"It is a myth that Iran is strong. The only reason Iran is strong is because of US weakness," he said. "Iran is very thinly stretched. It will not take a lot to contain Iran."
But even those celebrating the shift in American policy don't seem so sure.
"Tehran today is challenged by a strict, driven, strong and decisive US, which was not always the case with the lenient and hesitant Obama Administration," said a commentary yesterday in the Pan-Arab Asharq al-Awsat newspaper. "The region now faces turbulent winds of change. It will not be easy."
