Here's what to know about the long-strained relationship that poses the risk of a broader conflict.
US officials have suddenly raised the stakes in the long-simmering tensions between the United States and Iran, pointing to new intelligence that they say suggests an imminent threat to US interests in the Middle East.
The potential for armed conflict between the two countries has loomed since the Islamic Revolution and takeover of the US Embassy in Tehran four decades ago. Occasionally, those tensions have escalated into violence.
The recent US assertion that Iran poses an immediate threat has raised fears that the two nations have pushed closer to the brink. Here's what you need to know about the risks of a broader conflict.
How did we get here?
This month, the United States — pointing to information about an imminent threat of an Iranian attack in the Middle East — swiftly moved an aircraft carrier group into the region. In quick succession, it then shored up defences and evacuated personnel from the embassy in Baghdad, the Iraqi capital.
But the Trump administration has not provided specific details about the supposed threat from Iran, and allies in Europe and the region are skeptical given the history of faulty intelligence that led to the 2003 invasion of Iraq, led by the United States.
In response to the moves by the United States this month, Iran said it would end compliance with its obligations under the 2015 nuclear deal between Iran and six world powers. The deal with the United States, China, France, Germany, Russia and the United Kingdom was intended to curb Tehran's nuclear ambitions in exchange for relief from economic sanctions.
Tensions have risen steadily since the beginning of the Trump administration. President Donald Trump pulled the United States out of the nuclear deal last year, imposed oppressive sanctions, moved to cut off Iran's oil exports and designated an Iranian military unit as a terror organization.
Sanam Vakil, a senior research fellow in the Middle East and North Africa program at Chatham House, a London-based research group, said the Trump administration's lack of understanding about Iran has only fanned the flames.
"Something as simple as the very insulting language they use, which is political and by choice, is not language that works with the Islamic Republic of Iran," she said. "There's just very limited trust between both sides."
This isn't the war with Iraq. Here's why
While some have compared the situation with Iran to the lead-up to the U.S. invasion of Iraq, there are important differences.
Iran is nearly four times larger than neighboring Iraq in terms of territory. With a population of more than 80 million, it is the second-most populous country in the region after Egypt, with the second-largest economy in the Middle East and North Africa, according to the World Bank.
At the time of the US invasion of Iraq in 2003, the country had a Shiite Muslim majority but was governed by the Sunni Muslim minority. Iran's majority Shiite population is represented in government by its own sect.
Iran has a limited conventional military, but its growing network of proxy groups magnifies its influence in the region, meaning any conflict with US forces could result in a guerrilla war fought on multiple fronts.
The response from allies in Europe and the Middle East has also been very different from their response in the lead-up to the Iraqi war.
"There is a very clear divide with the trans-Atlantic allies as to how to handle this crisis," Vakil said. "Europe is prevailing upon the United States to be calm and measured in their response. You can very clearly see their frustrations that this is a manufactured crisis."
Regional allies — including the United Arab Emirates, Saudi Arabia and Israel, all of which have been supportive of the Trump administration's maximum pressure campaign on Iran — have stayed quiet so far.
"Their silence over the past few days is very telling that their bark was bigger than their bite," Vakil said. "By no means does the region want to absorb the pain of a conflict with Iran."
The relationship between Iran and the United States has teetered for decades
The tensions between Iran and the United States stretch back decades, well predating the Trump administration. The most dangerous flashpoint has long been the Strait of Hormuz, a narrow waterway that connects the Persian Gulf to the rest of the world.
It is the conduit for about 40% of the world's oil tanker traffic, and has been the recurrent backdrop of military bluster.
Iran warned it could close the strait during its war with Iraq, which lasted from 1980 to 1988, and has periodically threatened to mine the waterway since. The US Navy's 5th Fleet, which patrols the region from its base in Bahrain, has vowed to stop any menace to maritime traffic.
US and Iranian forces clashed in the strait repeatedly in 1988 after an Iranian mine damaged a Navy frigate. On April 18, 1988, the Americans sank three Iranian warships and destroyed two Iranian surveillance platforms in what was essentially a one-day war.
The deadliest confrontation between the two countries in the strait was in July 1988, when a US warship shot down an Iranian commercial airliner with two surface-to-air missiles, killing all 290 people aboard, including 66 children. The Americans said the crew mistook the plane for a fighter jet.
Other confrontations were not lethal but were seen as embarrassing.
In December 2011, Iranian forces claimed to have downed a sophisticated US surveillance drone that had gone missing. The United States said the drone had crashed and demanded Iran return it.
Instead, the Iranians put the drone on display and later claimed to have reverse-engineered it to produce their own.
In January 2016, days before the nuclear deal was to take effect, the Iranian Navy captured 10 US sailors whose patrol boats had strayed into Iranian territorial waters in the Persian Gulf. The Iranians took pictures of the sailors kneeling in surrender, in a move clearly meant to humiliate the United States.
The sailors were released after 24 hours, and a Defense Department inquiry concluded that Navy blundering was to blame.
So, what does Iraq have to do with this?
Iran and Iraq have grown increasingly aligned in the years since Saddam Hussein's ouster and the subsequent rise of a Shiite-led government.
Once fierce adversaries, the most recent national elections saw Iraqi parties with links to the paramilitary Popular Mobilisation Forces — an umbrella organisation of about 50 paramilitary groups, many with ties to Tehran, which successfully fought against the Islamic State group — gain even more power.
Iran has also moved to strengthen economic ties with Iraq, further deepening the relationship. This has left Iran and the United States in direct competition for influence in the country.
Last week, days after US officials first claimed an imminent threat from Iran, Secretary of State Mike Pompeo made an unannounced, hour long trip to Baghdad to press what he said was evidence of increased danger to Americans there. He met with the foreign minister, the prime minister and the president.
Officials told The New York Times that their concerns focused on Iran's Islamic Revolutionary Guards Corps and its links to some of Iraq's Shiite Arab militias, and said they had information to suggest plans to mobilize proxy groups in Iraq and Syria to attack American forces.
US officials later said that the intelligence included photographs of missiles on small boats in the Persian Gulf, put there by Iranian paramilitary forces. The United States feared they could be fired at its Navy.
Additionally, the US officials pointed to threats against commercial shipping and potential attacks by militias with Iranian ties on U.S. troops in Iraq. Iraqi officials this week warned the Iran-linked militias to refrain from provoking the Americans.
Written by: Megan Specia and Rick Gladstone
© 2019 THE NEW YORK TIMES