Image source, AHMAD AL-RUBAYE/AFP via Getty Images

There's been a lot of talk of de-escalation after President Trump and Iran's foreign minister each seemed to indicate they sought an off-ramp from the mounting conflict.

But some commentators are more sceptical.

In a series of tweets, external Yashar Ali - a US journalist of Iranian descent - said it was "naive" to think tonight's attack was the last of Iran's retaliation.

"The Iranian government has always operated on its own timeline," Ali wrote. "If you think Iran lobbing missiles over the border is the kind of revenge they ultimately have in mind, you're wrong."

"The Iranian gov sees revenge as almost a generational thing... their timeline can be in months/years," he said, listing examples of delayed or unexpected retaliation by Iran.

As Ali notes, the US and Iran share a long, and complicated history stretching back more than 65 years, from the CIA-orchestrated overthrow of Iran's prime minister in 1953, to tension and confrontation under President Trump.

Read more about the history of US-Iran relations here.

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Although Iran was targeting US troops, other countries have forces in Iraq as part of the anti-ISIS coalition.

Canada had announced on Tuesday that it was temporarily moving some of its about 500 troops to Kuwait as a precaution.

Following the rocket attacks, the military sent this tweet:

The UK foreign office told the BBC: "We are urgently working to establish the facts on the ground. Our first priority is the security of British personnel."

The UK has put the Royal Navy and military helicopters on standby amid rising tensions in the Middle East, Defence Secretary Ben Wallace said earlier.

Image source, Google Trends

In the hours after Iran's airstrike against the US, "Iran" became the top Google search in the US.

According to Google, Alaska is the most interested state today, followed by Oklahoma, Colorado, and West Virginia.

There have been more than two million searches for Iran in the US today.

This purports to show missiles landing at Al Asad air base.

There are still no reports of casualties but the US says it's still assessing the situation.

The Democrats who hope to take on - and beat - Donald Trump in the presidential election in November have been reacting to the latest developments.

Most have been critical of the drone strike ordered by the US president, questioning whether he has thought through the consequences.

But tonight, with US troops under attack, those who have ventured onto Twitter have stuck to sending messages of support.

Joe Biden tweeted that he wanted to wait until we know more before commenting. But a few hours earlier, the former vice-president was touting his foreign policy credentials compared to the current occupant of the White House.

You can read more below on how the Iran story could reshape the 2020 election campaign, from the BBC's Anthony Zurcher.

Could Iran strike help Trump get re-elected?

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Republican Senator Lindsey Graham said on Fox News this evening that Iran's attack on Iraqi bases housing US troops was "an act of war".

"The president has all the authority he needs under Article II to respond," the Trump loyalist said to host Sean Hannity, referring to Article II of the US Constitution which delineates presidential power.

"How he responds is yet to be determined, but he has that authority to respond."

Graham concluded by saying he had just spoken to President Trump, and appeared to threaten Iranian oil targets.

"Let me say tonight, if you are watching television in Iran, I just got off the phone to the president - your fate is in your own hands in terms of the regime's economic viability. You continue this crap, you're going to wake up one day out of the oil business."

Reporters in Washington have been reacting to the US president's first public response to the attack. With varying conclusions.

There's a lot going on tonight but it might be useful to take a step back and look at the wider picture too.

A SIMPLE GUIDE: If you want to know the basics, here's the story in 400 words

GO DEEPER: A brief history of US-Iran relations

PROFILE: Who was Qasem Soleimani?

READERS' QUESTIONS: Could this mean World War Three?

LEGAL TAKE: What does law say about killing Iran's top general?

ANALYSIS: Could this help Trump get re-elected?

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Iran's foreign minister Javad Zarif has tweeted that Iran does not "seek escalation or war, but will defend ourselves against any aggression".

"Iran took & concluded proportionate measures in self-defense under Article 51 of UN Charter targeting base from which cowardly armed attack against our citizens & senior officials were launched," Zarif said.

The tweet is being viewed as a potential sign of de-escalation on the part of Iran.

Speaking to CBS News earlier today, Zarif said he did not believe the US had chosen such a path.

So that's President Trump's first and long-awaited response to tonight's rocket attacks.

Let's look back at what he's said since Suleimani was killed.

On Sunday: “If it happens, it happens. If they do anything, there will be major retaliation”.

On Tuesday: "If Iran does anything that they shouldn't be doing, they're going to be suffering the consequences, and very strongly."

So, does the firing of a dozen ballistic missiles at US bases in Iraq cross a red line?

The US says it's still assessing the damage from the rocket attacks. But so far there have been no reports of American casualties - and analysts think that could be key to the president's thinking.

Liz Sly, Beirut bureau chief for the Washington Post, tweeted, external:

"This doesn’t yet feel like a major escalation. The US military was anticipating an attack and personnel had taken cover. Iran can claim it took revenge. Feels more like an escalation to de-escalate."

We've just had the US president's first public reaction to the attack.

Speaking to NBC News at an event for former Vice-President Joe Biden, former secretary of state John Kerry called for the Trump administration to respond to Iran with diplomacy, instead of a violent strike.

“I think it’s a tragedy for the world that instead of diplomacy, this administration has rushed to confrontation,” Kerry said.

“If this develops into a tit-for-tat, increased effort, it will become a war that is needless, it didn’t have to happen, and it will be a reckless war of choice by the president of the United States.”

The Al-Asad airbase - located in the Anbar province of western Iraq - was hit by at least six missiles in this evening's attack.

Iranian state TV reported that the elite Islamic Revolutionary Guards Corps struck the base, which houses US troops, "with tens of missiles".

US military officials had reportedly been anticipating a strike and had taken precautions. No causalities have been reported.

Trump himself visited the base in December 2018.

"I had concerns for the institutions of the presidency," Trump said at the time. "Not for myself personally."

"I had concerns for the First Lady," he added.

The president said the plane was darkened upon landing as a safety precaution, with no "lights on whatsoever".

"Pitch black," Trump said. "I've never seen anything like it."

Several veterans of the Obama White House have taken to Twitter to caution restraint in the US response to Iran's attacks.

Ilan Goldenberg, who oversaw Iran policy in the Pentagon under the Obama White House, insisted the president did not have to respond, writing on Twitter: “NO. NO. NO. He doesn’t have to respond especially if casualties are limited.”

Ben Rhodes, another Obama adviser, wrote: "There is still de-escalation as an available option."

Their comments were echoed by Philip Carter, a former Obama administration official who served in Iraq.

Image source, Getty Images

The Federal Aviation Administration has issued flight restrictions banning US civil aviation operators from flying over Iran, Iraq, and the waters of the Persian Gulf and the Gulf of Oman.

"We continue coordinating with our national security partners and sharing information with US air carriers and foreign civil aviation authorities," the FAA said in statement.

On Friday, just after news emerged that Soleimani had been killed in a drone strike ordered by the US president, Donald Trump tweeted out an image of an American flag.

On Tuesday night, just moments after Iran launched dozens of ballistic missiles against US targets in Iraq, Saeed Jalili tweeted an Iranian flag. He is reported to be close to Supreme Leader Ali Khamenei and has been one of Iran's top nuclear negotiators.

Image source, Getty Images

There had been speculation that President Trump would address the nation about the missile attacks but the White House says there will NOT be a presidential address.

Aides had reportedly been preparing remarks for the president to speak.

Top national security officials - including Secretary of State Mike Pompeo and Defense Secretary Mark Esper - departed the White House, US media report.

Vice-President Mike Pence has also left.

The long-running conflict between the US and Iran has escalated at dizzying speed in recent days. But what happens next, and how will President Trump respond now?

A few hours before Iran launched these rocket attacks the BBC's Katty Kay had asked two foreign policy experts on the Beyond 100 Days programme on BBC World News about whether it was realistic to expect Iran to de-escalate.

Two Iraqi bases housing US military forces were attacked - one at Al Asad and one in Irbil.

Al Asad air base was first used by American forces after the 2003 US-led invasion that toppled dictator Saddam Hussein, reports Associated Press news agency, and again during the fight against the Islamic State group. There are about 1,500 US and coalition forces there.

Iranian state TV says the attack is a retaliation after the country's top commander Qasem Soleimani was killed in a drone strike in Baghdad.

Soleimani was the head of the elite Quds Force - the overseas operations arm of Iran’s Revolutionary Guards - before his death on Friday. That organisation has also issued a statement carried by Iran's state-run IRNA news agency.

"We are warning all American allies, who gave their bases to its terrorist army, that any territory that is the starting point of aggressive acts against Iran will be targeted," it said via a statement.

The BBC's Middle East Quentin Sommerville has this profile of the man at the centre of this crisis, someone the Americans called a terrorists but who was treated as a rock star by his supporters.