"You can't do anything."

With those four words, Iran's supreme leader goaded Donald Trump into action.

It was mere hours after a mob of pro-Iranian protesters and militia members attacked the American embassy in Iraq's capital, Baghdad – surrounding it, pelting stones and setting fire to part of it.

Mr Trump, as is his wont, responded to the escalating events on Twitter, saying Iran would be held "fully responsible" for the attack and would pay a "very big price".

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"This is not a warning, it is a threat. Happy New Year!" he wrote.

Ayatollah Ali Khamenei replied with a provocative taunt.

According to The Times of London, it was at that point, with images of the burning embassy plastered all over TV, that Mr Trump "snapped" and settled on a drastic response he had previously ignored.

He decided to kill the notorious Iranian general Qassem Soleimani, a man second in power only to the supreme leader himself.

Soleimani was responsible for hundreds of American deaths. His own death has threatened to destabilise the entire Middle East.

It was the culmination of days of building tensions between Iran and the US.

On December 27, a rocket attack on an Iraqi military base killed an American civilian and injured several members of the US military.

The next day, Pentagon officials presented Mr Trump with a suite of options to consider in response to the attack. The most outlandish one was to kill Soleimani.

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A US airstrike near Baghdad's airport killed Qassem Soleimani. Photo / AP
A US airstrike near Baghdad's airport killed Qassem Soleimani. Photo / AP

"In the wars waged since the September 11 attacks, Pentagon officials have often offered improbable options to presidents to make other possibilities appear more palatable," The New York Times later reported.

The idea of killing Soleimani was one such option. The officials did not expect Mr Trump to choose it – and at first, he didn't. Instead the President ordered airstrikes on an Iranian-backed militia group.

The attack on the embassy, which marred New Year's Eve, helped changed his mind.

When Soleimani's killing became public, the Trump administration argued it was sparked by intelligence showing he was planning an imminent attack which could threaten hundreds of American lives.

It's unclear just how solid that intelligence was – some officials, speaking anonymously to the media, have described it as "razor thin".

But whatever the justification for Soleimani's death, the immediate consequence has been even greater friction between the two nations and confusion in the region.

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The Pentagon has scrambled to reinforce America's military presence in the Middle East. Iran has vowed to exact "severe revenge".

The situation has also created repercussions in Iraq, where this week, parliament voted to expel US troops from the country. Its caretaker Prime Minister Adil Abdul Mahdi described it as the only way to "protect all those on Iraqi soil".

Mourners attend a funeral ceremony for Iranian Gen. Qassem Soleimani and his comrades, who were killed in Iraq in a U.S. drone strike on Friday, in the city of Kerman, Iran. Photo / AP
Mourners attend a funeral ceremony for Iranian Gen. Qassem Soleimani and his comrades, who were killed in Iraq in a U.S. drone strike on Friday, in the city of Kerman, Iran. Photo / AP

In response, Mr Trump threatened to impose sanctions like Iraq has "never seen before".

But the resolution passed by parliament is not as clear cut as it sounds. It does not outline any specific timetable for American forces to leave, and because Iraq only has a caretaker government, it may not be legally binding.

Adding to that confusion, today an explosive letter, seemingly written by the head of US operations in Iraq, started to circulate in the media.

It said the Americans would be "repositioning forces" over the coming days and weeks to "prepare for onward movement" – or in plainer English, to prepare for a withdrawal.

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The US had to issue an awkward denial, saying the letter was a "draft" released by "mistake" and no withdrawal was happening.

"There is no decision to leave, nor did we issue any plans to leave or prepare to leave," Defence Secretary Mark Esper said.

The strike against Soleimani has also spawned a fierce political debate back in the US, with Mr Trump's Democratic opponents labelling the move "provocative and disproportionate".

Congressional leaders have complained that the President did not give them advance notice of the attack. They are due to receive briefings on the matter later this week.

House Speaker Nancy Pelosi said there were "serious and urgent" questions about "the timing, manner and justification of the administration's decision to engage in hostilities against Iran".

In response General Mark Milley, the chairman of the Joint Chiefs of Staff, insisted the intelligence behind the decision to kill Soleimani was solid.

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"It was imminent," he said of the Iranian general's plot.

"And it was very, very clear in scale and scope. Did it exactly say what, when and where? No, but he was planning, co-ordinating and synchronising significant operations against US military forces in the region, and it was imminent."