As the first signs of thaw between the United States and Iran arise, can decades of angry rivalry, vicious rhetoric, and violence give way to some form of détente?

In the days since presidents Barack Obama and Hassan Rouhani made a historic 15-minute phone call, the highest-level direct contact since before Iran’s 1979 Islamic revolution, both Tehran and Washington have shared an obsession with the word “trust” – specifically, how little trust each has in the other.

But is trust necessary – or even possible – after years in which both sides humiliated and undermined each other, racking up lists of grievances that have calcified into unbending narratives of the enemy? As Iran and the six world powers prepare to meet in Geneva next week to continue talks on Iran's controversial nuclear program, some portray any progress – more so than reaching a new level of trust – as a critical first step.

“The problem is both sides go in with these assumptions about the other side,” says John Limbert, a former senior US official who witnessed the sharp end of the US-Iran divide. He was among the 52 US diplomats held hostage in Tehran by militant students for 444 days from 1979-81.

That seminal crisis is but one of a long list of events that have kept both sides far apart and crying foul: a CIA-orchestrated coup in 1953, US support for Saddam Hussein in the 1980s Iran-Iraq war, a covert war to slow Iran’s nuclear program, and Iran’s targeting of Americans in Lebanon and more recently Iraq, along with steady threats to US-ally Israel.

Those actions “confirm what you already believe, [that] the other side is infinitely evil, duplicitous, and arrogant, and will do anything to harm you,” says Amb. Limbert, who was US deputy assistant secretary of state for Iran until 2010. “And if you look at statements and events over the past 10 or 20 years, you’re going to find things that are going to prove that.”

'Deception is part of the DNA'

Iran wants a deal within six months. When Mr. Rouhani spoke at the United Nations on Sept. 25, he said Iran “does not seek to increase tensions” with the US, and that both could “arrive at a framework to manage our differences.” He said his surprise election victory provided “a unique opportunity – for us all.”

Later, Rouhani told CNN that he had a message of “peace and friendship from Iranians to Americans,” who are “very near and dear to the hearts of the Iranian people.”

Yet a different tone came from Iran’s Supreme Leader on Saturday, when he said aspects of Rouhani’s New York diplomacy were “not appropriate” – an apparent reference to the Obama-Rouhani phone call.

“We are pessimistic about the Americans. We do not at all trust them,” Ayatollah Khamenei told graduating Army cadets, according to a translation on his official website. The US “is an arrogant, unreasonable and untrustworthy government which is completely under the influence of the international Zionist network.”

On the US side, Mr. Obama noted in his UN speech, also on Sept. 25, that US-Iran “mistrust has deep roots,” though a nuclear deal can be a “major step down a long road towards a different relationship, one based on mutual interests and mutual respect.” He assured the Iranians that the US is not seeking regime change.

As Iran’s top nuclear negotiator from 2003-2005, Rouhani negotiated a suspension of Iran's uranium-enrichment with the European Union. But at the same time, Iran increased its number of assembled centrifuges, though it did not use them to enrich. Rouhani has since stated that move was in keeping with the original deal, but critics see it as evidence of trickery.

When US Undersecretary of State for Political Affairs Wendy Sherman was asked while testifying before the Senate last week if the US trusted Rouhani after those events of a decade ago, she replied, “Senator, I don’t trust the people who sit across the table from me in these negotiations,”

Referring to that period, Sherman, who is the top US negotiator at the nuclear talks, said, “We know that deception is part of the DNA.”

A middle way?

Iranian officials indicate there may be a preferred middle way that can account for past grievances and recognize today’s mutual interests.

In his writings, Iran’s new Foreign Minister Mohammad Javad Zarif has said US ties would be useful, but can “never be friendly.” And in the past week, his deputy Abbas Araghchi struck a similar note.

“Definitely, a history of high tensions between Tehran and Washington will not go back to normal relations due to a phone call, meeting or negotiation,” said Mr. Araghchi, according to Iranian media. “We never trust America 100 percent. And, in the future, we will remain on the same path.”

US Secretary of State John Kerry stated last Thursday that it would be “diplomatic malpractice of the worst order” not to attempt a deal with Iran. However, he said, “nothing we do is going to be based on trust, but is going to be based on steps.”

But can either side accommodate worldviews that, almost by definition, require US-Iran rivalry and conflict?

“The question of trust boils down to 'what are the ultimate objectives of one side or the other?'” says an Iranian academic now in Washington, who asked not to be named.

One model may be Russia and the US, which cooperate and have embassies, but also “really have deep-seated differences,” says the Iranian academic.

“No matter how normalized the Islamic Republic becomes, being a revolutionary state, you can’t divorce that history from the Islamic Republic, of being the champions of the downtrodden, of the poor in the region,” adds the academic.

This also leaves space in Iran's political sphere for hardliners like Brig. Gen. Masoud Jazayeri, who have not hidden their distaste for Rouhani's US outreach. The senior Revolutionary Guard commander says the upcoming Nov. 4 anniversary of the 1979 seizure of the US Embassy will see the "Death to America" chant "resonate across the cities and villages of Islamic Iran."

“The crimes of the Americans and international Zionism in confronting the great nation of Iran will ever be erased from minds and memories,” Brig. Gen Jazayeri said.

Former hostage Limbert recalls a Persian proverb: “One crazy person, one fool, can throw a rock down the well. And a thousand smart people can’t get it out.”

“There are going to be setbacks,” says Limbert. “People are going to say stupid things, people are going to do stupid things. Are both sides good enough to keep focused on what they want to accomplish?”

Total recalibration

Both Obama and Rouhani face conservative backlash at home for even attempting to reach out. Those political forces have successfully torpedoed past engagement efforts.

“It’s very, very vulnerable, the situation. Each side expects the other to make an impressive move and [if] they don’t see it they [will] again fall back to, ‘I told you so, it won’t work, they are not sincere.’ It’s fragile,” says a veteran Iranian analyst in Tehran who asked not to be named.

“Some things send a very clear message, like when Obama said, ‘We are not after regime change,’ that was very big,” says the Tehran analyst. “This is a new chance, a new opportunity [with] a seriousness…the like of which I don’t remember for decades.”

For the Iranians, any turn toward the US also requires a recalibration, after 34 years of revolutionary anti-Americanism codified by chants of “Death to America” and flag burning among hardliners at Friday prayers and official rallies.

After toppling the pro-West Shah in 1979, revolution leaders vowed to be beholden to “neither East nor West.” US and Soviet flags alike were painted on streets to be walked and driven over in contempt. In recent years, Iran has also led an “Axis of Resistance” against US and Israeli influence in the Middle East, which includes Syria, Hezbollah in Lebanon, and Palestinian militant groups like Hamas and Islamic Jihad.

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In a fierce denunciation of the US last February, Khamenei listed a lifetime of grievances and ruled out direct talks.

“I believe the Leader is not ready” for US-Iran rapprochement, says an Iranian diplomat who asked not be named. “For 30 years we told the world that the US was the great enemy. If all of a sudden we reverse that, everything changes.”