With toppling of the pro-American regime led by Shah Mohammed Reza Pahlavi, the 1979 Iranian Revolution severed cordial relations between Washington and Tehran.
Relations between Washington and Tehran have been sour ever since the Iranian Revolution of 1979, bringing the two states to the brink of war on several occasions while engaging each other in bloody proxy battles.
But before 1979, the two countries were on good terms, even working against the former communist Soviet Union as allies.
The recent assassination of Iran's most powerful general Qasem Soleimani once again brought the two sides to the point of war, as Tehran vowed harsh revenge against Washington, which ackowledged it had carried out the killing on the orders of US President Donald Trump.
The Iranian retaliatory attacks that targeted the Iraqi bases hosting American forces caused no US casualties, an outcome that opened a way for de-escalation.
While a possible war has currently been avoided, bad blood continues to haunt the two archrivals, keeping tensions high in a volatile Middle East, where the US presence has not been welcomed much by indigenous populations.
Here's a quick recap on why the US and Iran have had rough relations in the past four decades.
The roots of anti-Americanism in Iran
Between 1941 and 1979, Iran, a strategically sensitive country between Central Asia and the Middle East, was led by a secular Persian monarch, Shah Mohammed Reza Pahlevi, a pro-Western leader.
The Shah's staunch pro-American stance enabled the ousting of nationalist Prime Minister Mohammed Mossadegh with the help of the CIA and MI6, British secret intelligence. Mossadegh wanted to nationalise the Anglo-Iranian Oil Company (AIOC) in 1953, a move that would have challenged British hegemony over the country's oil industry.
In the 20th Century, the British colonised Iran, exploiting the country’s rich oil resources and Mossadegh wanted to bring a certain end to that. But London, which owned 85 percent of AIOC’s profits whileTehran could just have 15 percent of the revenue, determined not that to happen.
In a bold military coup in August 1953, Mossadegh was ousted with the help of the US and Britain after a failed attempt earlier in the same month, when millions of pro-Mossadegh protesters strongly opposed the Western intervention, filling the streets of Tehran.
One of the main reasons for the fall of Mossadegh was the withdrawal of support by the powerful Shiite clergy, which backed the oil nationalisation but also feared the growing clout of the communists, who had allied with the nationalist prime minister.
Eventually, the Shiite religious establishment led by Ayatollah Abol-Ghasem Kashani, who was also the mentor of the future Ayatollah Ruhollah Khomeini, left Mossadegh vulnerable to the CIA-masterminded coup.
Mossadegh was punished with house arrest for his nationalist stance by the Shah until his death in 1967. He was even denied a public funeral despite his wish to be buried next to the killed protesters of 1952-53 in a public graveyard.
But Mossadegh’s anti-imperialist stance left a strong footprint on the country’s memory, and became one of the main rallying points in the 1979 Revolution against the Shah, who severely punished him for protecting the country’s interests.
Since then, Mossadegh has been regarded as one of the most respectful and popular politicians by Iranians in the country’s complicated history.
The 1979 revolution
Many experts think that the Shiite clergy has exerted more influence over the Iranian society than any other institutions in the country even though the Shah was the absolute monarch backed by the military.
“Shiite clerics have been able to mobilize [sic] the Iranian masses far better than any other socio-political authority. Clerics form the broadest social network in Iran, exerting their influence from the most remote village to the biggest cities,” wrote Mehdi Khalaji,an expert working for United States Institute of Peace.
The Shah’s unpopular economic policies and pro-American stances angered the Shiite clergy, led by the powerful and charismatic figure, Khomeini, who openly opposed the monarch’s modernisation programme, ending up in prison in 1963 after anti-government protests. The next year he was exiled by the Shah.
But Khomeini stayed a popular figure in Iran while he was living in exile and eventually ascended to the leadership of the revolution after an article in a Tehran newspaper led to the first protests in 1978.
As the protests grew further and the Shah’s security forces killed hundreds of demonstrators, the CIA was still thinking that Iran was not up to any kind of revolution, reassuring the Shah and its American counterparts.
“Iran is not in a revolutionary or even ‘prerevolutionary’ situation,” the CIA infamously declared five months before the Shah was ousted by revolutionary forces, which were a coalition of secularist liberals, communists and pro-clergy groups.
The then-US President Jimmy Carter was also so optimistic about Iran's future under the Shah.
"Under the Shah's brilliant leadership Iran is an island of stability in one of the most troublesome regions of the world. There is no other state figure whom I could appreciate and like more," he said, during a visit to Iran on December 31, 1977.
But after the revolution, Khomeini’s clergy established a powerful control over Iran’s political system, eliminating both secularists and communists following the Shah’s ousting.
Iran’s mullahs established a kind of semi-theocratic and semi-democratic system, installing a top post for the Shiite clergy, which is called the spiritual leadership, and allowing elections in which only candidates with a pass from the clergy could run for the presidency.
While the Iranian revolution in its political nature was anti-American, Tehran and Washington appeared to try to find a way to compromise with each other.
But when the Shah, who had been strongly identified with the US by revolutionary forces, landed in New York City for medical treatment in October 1979, suspicions over Washington’s intentions towards Iran increased, triggering an attack on the American embassy.
In early November, the attackers, most of whom were hardcore Khomeini supporters, took 52 American embassy staffers as their hostages for 444 days in a stalemate which sent shock waves around the world, making both countries bitter enemies.
Forty years later, Trump threatened Tehran after killing its Revolutionary Guards’ Quds Force Commander Soleimani, saying that Washington could hit 52 Iranian cultural sites, alluding the number of hostages Iranians took during the 1979 revolution.
While all the hostages were eventually released by Iranians, the incident left an indelible mark on US-Iran relations.
1983 Lebanon bombings
Since 1979, Iran’s new state has developed a wide network of proxies across the Middle East to export its revolutionary ideology to other Muslim countries, making itself an influential power in the region.
Lebanon, which has a sizeable Shiite population, was one of Tehran’s targets to extend its reach in the Middle East, where it helped establish Hezbollah, a political group with an armed wing.
In 1983, both the US Embassy and the country’s peacekeepers in Beirut had been targeted by allegedly Hezbollah, killing more than 250 Americans in total.
Hezbollah’s alleged involvement contributed to worsening relations between the two countries, leading the US to designate Iran the following year as a state sponsor of terrorism, putting harsh sanctions on the country.
US hits Iran Air Flight 655
In 1988, as the US and Iranian naval forces were confronting each other in the Gulf, an American warship hit a civilian Iranian passenger, killing all 290 on board. Most of them were pilgrims on the way to Mecca.
While Americans claim it was a mistake, Iran maintained the attack was intentional. Washington has never apologised for the attack, angering Iranians until now.
After Trump threatened to attack 52 cultural sites, Iranian President Hassan Rouhani mentioned the number 290 for the possible Tehran retaliation.
During the brutal war between the two neighbouring countries from 1980 to 1988, Washington sided with Iraq, providing the former Saddam Hussein regime essential intelligence and military supplies, feeding Iranian anti-Americanism further.
American administrations also authorised the sale of deadly chemicals, which included biological viruses like anthrax and the bubonic plague, to Baghdad.
More US sanctions
In the 1990s, the US Congress imposed more sanctions on Iran, passing several laws to punish Tehran. The then-President Bill Clinton also issued executive orders to expand sanctions against Iran.
Iran’s nuclear programme
While Washington helped Tehran launch its nuclear programme in the first place, after 1979, it became a menacing issue between the two countries.
Iran’s uranium enrichment programme has been seen by the US, Israel and their allies as an effort to produce nuclear weapons. Tehran denied that saying it was for civilian purposes.
During the Barack Obama administration, Washinton aimed to develop an understanding with Tehran, eventually creating a mechanism called the Joint Comprehensive Plan of Action (JCPOA) to limit Iran’s nuclear programme in exchange for easing economic sanctions against Tehran.
In addition to the US and Iran, the co-signatories of the JCPOA were the UK, France, Russia, China and Germany – the permanent members of the UN – as well as the EU.
But Trump withdrew from the deal in 2018 and imposed harsh sanctions on Tehran, brewing tensions between the countries again.
In 2019, a series of attacks began happening in the Gulf, targeting oil tankers. The US accused Iran of the attacks.
In June, Iran shot down an expensive US military drone over the strategic Strait of Hormuz in the Gulf, escalating tensions.
Soleimani’s bold moves and his assassination
In the wake of escalating tensions, Iran’s most daring and powerful general, Soleimani, a man, who was the embodiment of the revolution’s ideology and Persian strategic thinking, increased his pressure on the US, allegedly leading attacks on a US base in Kirkuk and the US Embassy in Baghdad.
Trump, who did not respond to Iran’s attacks in the Gulf and an alleged Iranian strike to Saudi oil facilities, unexpectedly decided to killSoleimani in January, ordering a drone attack to the general in Baghdad.
As Iran lost its most valuable general, the country wowed for a stiff retaliation, edging both countries to the brink of war. But the Iranian retaliation against Iraq bases hosting American forces did not kill any US soldiers.
After the Iranian strikes, Trump tweeted “all is well!”, de-escalating the conflict.