The US-Iran conflict: A timeline of how we got here

The US helps stage a coup to overthrow Iran’s democratically elected prime minister, Mossadegh. He had moved to nationalize the country’s oil fields -- a move the US and Great Britain saw as a serious blow, given their dependence on oil from the Middle East. After toppling Mossadegh, the US supports Iran’s monarch Mohammad Reza Pahlavi to rule as Shah of Iran. Iranians resent the foreign interference, fueling anti-American sentiment in the country for decades to come.

The US signs a civil nuclear cooperation agreement with Iran. The agreement provides Iran with technology and resources that eventually become the foundation for its controversial nuclear program, which it begins developing in the 1970s with support from the US.

Millions of Iranians take to the streets in protest of the Shah’s regime, which they view as corrupt and illegitimate. Secular protesters oppose his authoritarianism, while Islamist protesters oppose his modernization agenda. On January 16, the Shah flees the country. On February 1, Ayatollah Khomeini, an Islamic scholar who had been arrested and deported by the Shah in 1964, returns from exile and becomes the supreme leader of the Islamic Republic.

Iranian students storm the US embassy in Tehran and take dozens of Americans hostage. They demand the Shah, who had been admitted into the US for cancer treatment, be extradited to Iran to stand trial for “crimes against the Iranian people.” After 444 days, Iran releases the hostages in exchange for state assets being unfrozen --  minutes after President Ronald Reagan is sworn into office. During the crisis, the US cuts all diplomatic ties with Iran. Formal diplomatic relations have never been restored. The Shah dies in July 1980 in Cairo.

The bad blood between the two countries is only made worse when the US backs Iraq in its invasion of neighboring Iran, prompting an eight year regional war. Qasem Soleimani, who has joined the elite Iran Revolutionary Guard Corps by this time, fights on the frontlines of this war.

Under Reagan’s administration, the US declares the Islamic Republic a “state sponsor of terrorism.” Decades later, this designation by the US still stands.

While the US is backing Iraq in its war with Iran, President Reagan’s administration covertly begins looking into ways to improve its relationship with Iran. The press reveals that despite an embargo on selling weapons to Iran, Reagan approved a high-profit sale in hopes that it will yield the return of Americans held hostage in Lebanon by Hezbollah, a militia with close ties to Iran.

While American and Iranian ships are exchanging fire in the Persian Gulf, the US mistakes a civilian airliner for a fighter jet and shoots down Iran Air Flight 655. All 290 passengers and crew members on board are killed. Though the US says the attack is an accident, Iranians see it as intentional.

Qasem Soleimani, who has been rising through the ranks of the Iran Revolutionary Guard Corps, is appointed chief commander of the Quds Force, an elite special forces unit that handles Iran’s overseas operations. He is instrumental in spreading Iran’s influence in the Middle East, and soon becomes one of the country’s most powerful leaders.

After the 9/11 attacks, Iran quietly helps the US in its war against the Taliban, a mutual enemy of both countries. But in a State of the Union address, President George Bush refers to Iran, along with Iraq and North Korea, as part of an “Axis of Evil.” The speech incites anger in Iran.

As the US voices concerns that Iran is attempting to develop nuclear weapons, inspectors from the International Atomic Energy Agency announce they’ve found traces of highly-enriched uranium at a nuclear plant in Iran. Tehran agrees to suspend production of enriched uranium and allow stricter inspections of its nuclear sites, but this is short-lived. President Mahmoud Ahmadinejad would come into power a few years later and restart Iran’s production of enriched uranium, prompting years of international sanctions against the country.

After years of negotiations with President Barack Obama’s administration, six nations and Tehran reach a landmark agreement that slows Iran’s nuclear development program in exchange for lifting some sanctions that caused the country’s economy to stagnate. It’s a huge breakthrough for the US and Iran, which have long been at odds.

A week after his inauguration, President Donald Trump signs an executive order banning nationals from seven Muslim-majority nations, including Iran, from entering the US for 90 days. Iran calls the ban “an obvious insult to the Islamic world” and responds by conducting a ballistic missile test. The back-and-forth marks a sudden escalation in tensions between the two countries, raising concerns about the future of the Iran nuclear deal.

Trump fulfills a campaign promise and announces he is withdrawing the US from the Iran nuclear deal, which he viewed as “one-sided.” He also says he will place new sanctions on the regime. Critics warn the move could lead Iran to restart its atomic program and set the stage for more conflict in the Middle East.

President Trump announces that the US will formally designate the Iran Revolutionary Guard Corps, Tehran’s most powerful military institution, as a foreign terrorist organization. The move is unprecedented, marking the first time that the US designated a part of another government as a terror group. Iran responds by declaring the US a “state sponsor of terrorism.”

Tensions further escalate after attacks on oil tankers in the Persian Gulf and oil facilities in Saudi Arabia, as well as the downing of a US drone, which Washington and its allies blame on Iran. American officials repeatedly stress that threats to the freedom of navigation in the Gulf and the oil trade are unacceptable, but the Trump administration takes no action.

After the US imposes several rounds of sanctions on Iran that were originally lifted under the Iran nuclear deal, Iran announces it will ramp up enrichment of low-grade uranium. This breaks the stockpile limit it agreed to in 2015, and brings the country one step closer to being able to build a nuclear bomb.

US forces conduct airstrikes on facilities in Iraq and Syria, which the Pentagon claims are linked to pro-Iranian militias responsible for attacking US service personnel in Iraq. At least 25 people are killed, an Iran-backed militia says. Days later, hundreds of pro-Iranian demonstrators attempt to storm the US embassy in Baghdad, scaling the walls and forcing the gates open, in protest of the US airstrikes.

Trump says he ordered an airstrike in Iraq to kill Iran’s top general, Qasem Soleimani -- a move that previous US presidents had considered far too provocative. Thousands of people flood the streets of Iran to mourn him. Iran is furious and vows retaliation, ultimately firing missiles at Iraqi bases that house American troops a few days later. No lives are lost, and Trump responds by promising more sanctions.

As tensions rise, Iran mistakenly shoots down a Ukrainian passenger jet, attributing it to a fear of US aggression. All 176 people on board are killed.

Trump’s decision to respond diplomatically to Iran’s missile attack and not militarily signals that tensions may cool off -- at least for now.