Mounting tensions between Washington and Iran have raised fears of a military confrontation – but how did we get here?

Ever since the United States tore up a multilateral deal on Iran’s nuclear programme last year, tensions between the two countries have been on the rise.

The administration US President Donald Trump has since reimposed harsh sanctions to strangle the Iranian economy, while Tehran has taken a series of steps to scale back commitments to the 2015 accord.

Tensions escalated further last week following US accusations that Iran was behind attacks that set ablaze two major oil installations in eastern Saudi Arabia. Tehran denied involvement.

With the US-Iran friction dominating global headlines as world leaders meet this week at the United Nations General Assembly, Al Jazeera takes a look at the key events that have led to the current situation.

2018

US withdraws from Iran nuclear deal

Trump makes good on an election campaign promise, announcing on May 8 that the US is withdrawing from the Iran nuclear deal, formally known as the Joint Comprehensive Plan Of Action (JCPOA).

“I made clear that if the deal could not be fixed, the United States would no longer be a party to the agreement,” Trump says.

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“The Iran deal is defective at its core.”

The JCPOA had tightly restricted Iran’s nuclear programme in return for ending sanctions that had severely damaged its economy.

In response, Iran calls Trump’s “unacceptable” and says it will bypass Washington and negotiate with the deal’s other remaining signatories: France, Germany, the United Kingdom, Russia and China.

US sets outs tough demands

The US on May 21 demands Iran make sweeping changes – from dropping its nuclear programme to pulling out of the Syrian war – or face severe economic sanctions.

The Trump administration’s 12 demands, which are outlined by US Secretary of State Mike Pompeo, are rejected by Tehran.

First round of sanctions

The US on August 7 reimposes the first round of sanctions on Iran, originally lifted as part of the nuclear deal.

They prohibit trade with a number of business sectors – from aviation and carpets to pistachios and gold.

Second round of sanctions

On November 5, the US announces a new round of sanctions, this time specifically targeting the key oil and banking sectors.

2019

‘Foreign terrorist organisation’

On April 8, Trump announces he is designating a powerful arm of the Iranian military, the elite Islamic Revolutionary Guard Corps (IRGC) a foreign terrorist organisation.

It is the first time Washington has formally labelled another country’s military a “terrorist group.”

The designation imposes wide-ranging economic and travel sanctions on the IRGC that go into effect on April 15.

Responding to the move, Iran immediately declares the US a “state sponsor of terrorism” and calls Washington’s forces in the region “terrorist groups.”

Iran’s Revolutionary Guard troops march in a military parade just outside Tehran, Iran [File: Ebrahim Noroozi/AP]

US sends aircraft carrier to the Middle East

On May 5, Trump’s National Security Adviser John Bolton announces the US is sending an aircraft carrier strike group and Air Force bombers to the Middle East “in response to a number of troubling and escalatory indications and warnings”.

“The United States is not seeking war with the Iranian regime, but we are fully prepared to respond to any attack, whether by proxy, the Islamic Revolutionary Guard Corps or regular Iranian forces,” Bolton says.

More sanctions

On May 8, Iran says it is preparing to increase enriched uranium and heavy water production as part of its decision to stop certain commitments made under the nuclear deal.

A year after Washington withdrew from the deal and later reimposed sanctions on Tehran, Trump announces new measures against Iran’s steel and mining sectors.

Starting today, Iran does not keep its enriched uranium and produced heavy water limited. The EU/E3+2 will face Iran's further actions if they can not fulfill their obligations within the next 60 days and secure Iran's interests. Win-Win conditions will be accepted. — Hassan Rouhani (@HassanRouhani) May 8, 2019

Tankers ‘ subjected to sabotage operations’

On May 12, the United Arab Emirates says four commercial ships off the coast of Fujairah, one of the world’s largest bunkering hubs, “were subjected to sabotage operations”.

Officials identify the damaged ships as the Saudi oil tankers Al-Marzoqah and Amjad, the Norwegian tanker Andrea Victory, and a UAE bunkering barge, the A Michel.

Fujairah is the only Emirati terminal located on the Arabian Sea, bypassing the Strait of Hormuz through which most Gulf oil exports pass.

Iran, which has repeatedly threatened to close the Strait in case of a military confrontation with the US, calls the incidents “alarming and regrettable”.

Houthis attack oil pipeline

Yemen’s Houthi rebels, who are locked in a long-running war with a Saudi-UAE-led military coalition, launch drone attacks on Saudi Arabia on May 14, striking a major oil pipeline and taking it out of service.

Two days later, Riyadh, a key US ally, blames Iran for the attack.

The US and Saudi Arabia accuse Iran of arming the Houthis, but Tehran denies the claim.

‘Never threaten the US’

On May 19, a rocket lands near the US embassy in Baghdad. No one is harmed.

It is not clear who is behind the attack, but Trump tweets: “If Iran wants to fight, that will be the official end of Iran. Never threaten the United States again!”

If Iran wants to fight, that will be the official end of Iran. Never threaten the United States again! — Donald J. Trump (@realDonaldTrump) May 19, 2019

Iranian Foreign Minister Mohammed Javad Zarif responds by saying Trump had been “goaded” into “genocidal taunts”.

Goaded by #B_Team, @realdonaldTrump hopes to achieve what Alexander, Genghis & other aggressors failed to do. Iranians have stood tall for millennia while aggressors all gone. #EconomicTerrorism & genocidal taunts won't "end Iran". #NeverThreatenAnIranian. Try respect—it works! — Javad Zarif (@JZarif) May 20, 2019

After meeting Japanese Prime Minister Shinzo Abe, who offers to broker dialogue between Washington and Tehran, Trump says on May 27 the US is “not looking for regime change” in Iran.

Shinzo Abe in Tehran

On June 12, Abe arrives in Tehran in a bid to mediate between the US and Iran.

A day later, he meets Iran’s Supreme Leader Ayatollah Ali Khamenei who tells him: “I don’t consider Trump as a person worthy of exchanging messages with. I have no response for him and will not answer him.”

New shipping incident

On June 13, with Abe still in Iran, a Japanese and a Norwegian tankers come under “attack” in the Gulf of Oman, according to the Norwegian maritime authority and the Japanese shipowner.

The US Fifth Fleet says it received two separate distress calls from the tankers in a “reported attack”.

Iran speaks initially of “accidents” and says it rescued 44 crew. Zarif calls tanker “attacks” during Abe’s visit “suspicious”.

More US troops in the region

On June 17, the Pentagon authorises the deployment of 1,000 additional troops to the Middle East.

On the same date, Iran says it is 10 days away from surpassing the limits set by the nuclear deal on its stockpile of low-enriched uranium.

Iran says it can reverse the move if the deal’s European signatories step in and make an effort to circumvent US sanctions.

US Drone is shot down

On June 20, Iranian forces shoot down a US military drone.

Both countries confirm the incident but offer diverging accounts about the location of the aircraft.

The US says it was flying above international waters, while Iran says the drone was flying in Iranian airspace.

Trump says he has called off an attack

On June 21, Trump says he called off a military strike on Iran the night before, which was intended as retaliation against Tehran for the downing of the unmanned US drone.

Trump says he did so 10 minutes before the planned attack because of potential casualties, saying it was “not proportionate to shooting down an unmanned drone”.

Trump says a US strike could have killed 150 people, and signals he is open to talks with Tehran.

….proportionate to shooting down an unmanned drone. I am in no hurry, our Military is rebuilt, new, and ready to go, by far the best in the world. Sanctions are biting & more added last night. Iran can NEVER have Nuclear Weapons, not against the USA, and not against the WORLD! — Donald J. Trump (@realDonaldTrump) June 21, 2019

Iran to ‘confront and threat’

On June 22, Iran says it is ready to respond firmly to any US threat against it.

“We will not allow any violation against Iran’s borders. Iran will firmly confront any aggression or threat by America,” Abbas Mousavi, foreign ministry spokesman, says.

On the same day, Iran orders the execution of a “defence ministry contractor” convicted of spying for the US Central Intelligence Agency, while the US vows to impose fresh sanctions, adding that military action was still “on the table.”

New US sanctions

On June 25, Trump signs an order targeting Khamenei, Iran’s supreme leader, and associates with additional financial sanctions

“Sanctions imposed through the executive order … will deny the supreme leader and the supreme leader’s office, and those closely affiliated with him and the office, access to key financial resources and support,” the US president says.

Responding to the announcement, Zarif, the Iranian foreign minister, tweets that hawkish politicians close to Trump were thirsty for war rather than diplomacy.

.@realDonaldTrump is 100% right that the US military has no business in the Persian Gulf. Removal of its forces is fully in line with interests of US and the world. But it's now clear that the #B_Team is not concerned with US interests—they despise diplomacy, and thirst for war. — Javad Zarif (@JZarif) June 24, 2019

Rouhani dismisses the sanctions as “outrageous and idiotic”, adding that Tehran’s “strategic patience” should not be mistaken for fear.

US deploys F-22 stealth fighters

On June 29, the US Air Forces Central Command says in a statement that F-22 Raptor stealth fighters are being deployed in the region “to defend American forces and interests”.

Tehran exceeds uranium limit

On July 1, Iran exceeds the limit on the amount of enriched uranium in its stockpile set out in the nuclear deal.

The United Nations’ atomic watchdog confirms that its inspectors had verified the 300kg cap had been breached.

Zarif says the accumulation of more enriched uranium than permitted under the deal is not a violation of the pact.

Tanker carrying Iranian oil stopped

On July 4, British Royal Marines, police and customs agents in Gibraltar seize a supertanker accused of carrying Iranian crude oil to Syria in breach of European Union sanctions.

The Grace 1 vessel was boarded on Thursday when it slowed down in a designated area used by shipping agencies to ferry goods to ships in the UK territory along Spain’s southern coast.

Iran passes new nuclear deal limit

On July 8, Iran passes the uranium enrichment cap set in the nuclear deal, the second time in a week that it makes good on a promise to reduce compliance with the accord.

Captain of Iranian vessel arrested

On July 12, police in Gilbratar arrest the captain and chief officer of an Iranian tanker that was seized by British forces the previous week.

Iran seizes British oil tanker

On July 19, the IRGC says its forces have seized a British oil tanker in the Strait of Hormuz.

The Stena Impero tanker “was confiscated by the Revolutionary Guards at the request of Hormozgan Ports and Maritime Organisation when passing through the Strait of Hormuz, for failing to respect international maritime rules”, the force says in its official website.

British navy to escort all UK vessels in Strait of Hormuz

On July 25, the UK announces the country’s warships will escort all British-flagged vessels through the Strait of Hormuz, a change in policy that takes place amid rising tensions in the Gulf.

HMS Montrose, a British frigate, has been tasked to sail alongside the ships for protection.

“Freedom of navigation is crucial for the global trading system and world economy, and we will do all we can to defend it,” a UK government spokesperson says.

US sanctions Zarif

On August 1, the US imposes sanctions on Zarif for acting on behalf of Ali Khamenei.

“Javad Zarif implements the reckless agenda of Iran’s Supreme Leader, and is the regime’s primary spokesperson around the world,” Treasury Secretary Steven Mnuchin says in a statement.

Zarif brushes off the move on Twitter, saying it indicates Washington saw him as a “threat”.

“It has no effect on me or my family, as I have no property or interest outside of Iran,” he says.

The US' reason for designating me is that I am Iran's "primary spokesperson around the world"

Is the truth really that painful?

It has no effect on me or my family, as I have no property or interests outside of Iran.

Thank you for considering me such a huge threat to your agenda. — Javad Zarif (@JZarif) July 31, 2019

Seized Iranian tanker free to sail

On August 15, Gibraltar’s Supreme Court rules that the Grace 1 is free to sail, just hours after the US makes a last-minute attempt to keep the vessel under detention.

Iran unveils new missile defence system

On August 23, Rouhani inducts a locally built air-defence system into the country’s missile defence network at an unveiling ceremony in Tehran.

Iran began production after the purchase of Russia’s S-300 system was suspended in 2010 due to international sanctions that have barred it from importing many weapons.

Speaking at the ceremony, Rouhani says the mobile surface-to-air system was “better than S-300 and close to [more advanced] S-400”.

Zarif meets Macron

On August 26, Iran‘s top diplomat holds talks with France‘s President Emmanuel Macron at the sidelines of a G7 summit following a surprise invite to the gathering in Biarritz.

“Iran’s active diplomacy in pursuit of constructive engagement continues,” Zarif says. “Road ahead is difficult. But worth trying.”

On the same day, Iran says it has sold 2.1m barrels of crude oil on board the tanker that was seized in Gibraltar the previous month, adding that the vessel’s new owner will decide on its next destination.

Iran further exceeds nuclear-deal limits

On August 30, the UN says Iran is still exceeding limitations set by its nuclear deal with world powers, increasing its stock of enriched uranium and refining it to a greater purity than allowed in the agreement.

The quarterly report from the UN’s International Atomic Energy Agency confirms Iran is progressively backing out of the pact in retaliation for the US’s withdrawal from the accord and the subsequent renewal of sanctions that have hit Iranian oil sales.

Sanctions on Iran’s space agencies

On September 3, the US imposes sanctions on Iran‘s civilian space agency and two research organisations, saying they were being used to advance Tehran’s ballistic missile programme.

The measures imposed by the US Department of the Treasury target the Iran Space Agency, Iran Space Research Center and the Astronautics Research Institute.

“The United States will not allow Iran to use its space launch program as cover to advance its ballistic missile programs,” Pompeo, the US secretary of state, says.

US intensifies pressure

On September 4, the US turns up the economic pressure on Iran, blacklisting an oil shipping network that Washington alleges is directed by the IRGC.

The blacklisted group of firms, ships and individuals stands accused by the US Treasury of breaching sanctions by supplying Syria with oil worth hundreds of millions of dollars.

The Trump administration, meanwhile, says it will not accommodate a proposal by France to throw a financial lifeline to Tehran.

US offers money to captain of Iranian tanker

The US offers several million dollars to the Indian captain of an Iranian oil tanker suspected of heading to Syria, the State Department confirms.

The Financial Times reports on September 5 that Brian Hook, the State Department point man on Iran, has sent emails to captain Akhilesh Kumar in which he offered “good news” of millions in US cash to live comfortably if he steered the Adrian Darya 1, formerly known as Grace 1, to a country where it could be seized.

Iran activates advanced centrifuges

On September 7, Iran starts injecting gas into advanced centrifuges to increase its stockpile of enriched uranium and warns time is running out for the nuclear deal’s other signatories to save the landmark pact.

Spokesman Behrouz Kamalvandi says Iran’s Atomic Energy Organisation has started up advanced centrifuges at the enrichment facility in Natanz, the third step by Tehran in scaling back its commitments under the crumbling pact following Washington’s withdrawal.

Bolton fired ‘over Iran’

Trump on September 10 announces via Twitter that he has fired Bolton, his national security adviser, saying he has “strongly disagreed” with many of his hawkish positions.

Bolton’s sacking is reportedly linked to a fundamental disagreement over the possible easing of US sanctions on Iran.

Taking aim at Bolton, Iran says the US should distance itself from “warmongers”.

Iran rejects accusation over Aramco attack

On September 14, Yemen’s Houthi rebels claim responsibility for drone attacks on two major Saudi Aramco oil facilities: Abqaiq – the world’s largest oil processing plant – and the Khurais oilfield, in eastern Saudi Arabia. The pre-dawn strikes knock out more than half of crude output from the world’s top exporter.

Pompeo swiftly blames Iran, saying it “has now launched an unprecedented attack on the world’s energy supply”.

“There is no evidence the attacks came from Yemen,” Pompeo says on Twitter, referring to the Houthis’ claim of responsibility. He does not provide any evidence to support his claim.

Iran dismisses the “meaningless” US allegations, saying they were meant to justify actions against the country.

Trump lashes out at Iran at UNGA

Addressing the UN General Assembly in New York, Trump on September 24 lashes out at Iran and calls countries around the world to tighten the economic noose around it.

“One of the greatest security threats facing peace-loving nations today is the repressive regime in Iran,” he says.

“The regime’s record of death and destruction is well known to us all. Not only is Iran the world’s number one state sponsor of terrorism, but Iran’s leaders are fuelling the tragic wars in both Syria and Yemen, and at the same time the regime is squandering the nation’s wealth and future in a fanatical quest for nuclear weapons.”