BBC
Iran-US hostility eased under President Obama but is threatening to intensify again
Are the US and Iran heading for a new confrontation? After a turbulent first three weeks in which President Donald Trump described Iran as "the world's number one terrorist state" and put it "on notice", it is a question many are asking.
For Iranians with connections in the United States, these are worrying times.
Of the seven majority Muslim countries named in President Trump's January travel ban (frozen pending a legal review), Iran is the one with the largest US-based diaspora, the most overseas students and the highest number of people travelling on visitor visas.
After the ban was announced, BBC Persian received hundreds of messages from anxious Iranians whose lives have been plunged into uncertainty.
They come from all walks of life - research students, LGBT (lesbian, gay, bisexual and transgender) refugees and grandparents on family visits - and many are worried the story is far from over.
"Last year our family applied to migrate to the US," wrote Bardia, a 16-year-old from the persecuted Bahai religious minority. "Now there's a big hold-up in the process."
But since President Trump moved into the White House it is not just Iranians with travel plans who are feeling unsettled.
Across the country people are asking themselves if he will really deliver on his promise to "rip up" the 2015 Iran nuclear deal and "triple-up" sanctions.
And if the war of words between Washington and Tehran continues, what will the impact be on Iran's presidential elections this May?
Deal in danger?
On the campaign trail Donald Trump dismissed the Iran nuclear deal as "disastrous", but Iran experts say comments by his new Secretary of Defence James Mattis are probably the best indicator of what lies ahead.
"I think it is an imperfect arms control agreement," Mr Mattis told a Senate committee in January. "But when America gives her word, we have to live up to it."
It is possible the Trump administration could push to toughen up the deal, says Gary Samore, former Obama White House Co-ordinator for Arms Control,
"But they will quickly find out any renegotiation of the agreement will require the US to offer additional sanctions relief."
Isfahan uranium conversion facility (file photo)Image copyrightAP
Image caption
The Isfahan uranium conversion plant - the US has vowed to robustly police the nuclear deal
Many point out the US is not the only signatory to the deal.
If Mr Trump walks away he will risk alienating the European Union, the United Kingdom, France, Germany, China and Russia, which would make enforcing any new sanctions more difficult.
But there are more subtle ways of undermining the agreement, says Nader Hashemi, of the Centre for Middle East Studies at the University of Denver.
"I suspect Trump will try to strictly enforce the nuclear deal, hoping that Iran will break the agreement and thus be blamed internationally for it."
Filip to hardliners
Iran's Supreme Leader Ayatollah Khamenei and his hardline supporters have been relatively muted in their response to President Trump.
It has lead some to suggest it might actually suit them to have a more confrontational president in the White House.
For someone used to rallying his supporters with denunciations of the "Great Satan", Mr Khamenei clearly feels on familiar ground responding to tougher rhetoric from Washington.
"We appreciate Trump! Because he largely did the job for us in revealing true face of America," he Tweeted recently.
Some hardliners actually see Mr Trump as a man they could do business with, says Mohsen Milani, an Iran specialist at the University of South Florida.
"They believe he is a practical, non-ideological businessman and a good deal-maker who would be willing to negotiate with Tehran."
Elections impact
One person for whom Mr Trump's ascendancy is less welcome is President Hassan Rouhani.
He is standing for re-election in May, and the accelerating war of words between Washington and Tehran casts a long shadow over his two biggest achievements - securing the nuclear deal and improving relations with the US.
As the election campaign gets under way Mr Rouhani's hardline opponents will seek to use the Trump administration's actions to undermine him.
Hassan Rouhani (10/02/17)Image copyrightREUTERS
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US-Iran tensions could work against President Rouhani's re-election bid
"If the [travel] ban is a sign of a general line towards Iran with additional measures, then it certainly could affect the elections," says Trita Parsi, of the National Iranian-American Council.
But whether the hardliners will succeed is open to question.
Even if the tangible benefits of the end of sanctions have yet to be widely felt in Iran, the prospect of the country returning to the international stage and opening up for business has given hope to millions of ordinary voters.
It is clear they do not want to see these achievements reversed.
Since Mr Trump's travel ban thousands of young Iranians have taken to Twitter using the hashtag #LoveBeyondFlags to reach out to Americans.
And among the traditional anti-American slogans on display at the annual rally to commemorate the Revolution in Tehran last week there were some in English with a rather different message: "Americans are welcome and invited to Iran".
Courting Russia
In the months to come foreign policy concerns will also influence US-Iranian relations.
Both the US and Iran are currently supporting Iraqi forces in the crucial battle to recapture Iraq's second city of Mosul from so-called Islamic State. It is not in the interests of either side to jeopardise this.
If President Trump delivers on his pledge to mend fences with Russia that could also impact on the US relationship with Iran.
Qadr H long-range ballistic surface-to-surface missile fired in Iran (09/03/16)Image copyrightAP
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Trump has warned Iran over its long-range missile tests (file photo)
"Tehran's biggest fear is that Trump will seek to move Russia away from Iran in order to open space for Russia-America co-operation in Syria and across the Middle East," says Alex Vatanka of the East-West Institute.
Going forward there will be many possible flashpoints for tension between Iran and the US.
Iranian ballistic missile tests, more unilateral US sanctions, stand-offs between the Iranian and US navies in the Gulf, and between US-backed and Iran-backed militia forces in Iraq will all test the relationship.
"Over time the [nuclear] deal may unravel because of these," says Gary Samore.
"But I think it's unlikely either side will immediately abrogate the agreement. The US benefits from the constraint on Iran's nuclear programme and Iran benefits from the sanctions relief."
