Iran crisis: How and why a British oil tanker was seized by Iran's Revolutionary Guards

“Allahu akbar”, or God is great, the Iranian Revolutionary Guard marine was heard shouting off camera as the group took control of the British-flagged Stena Impero.

Scaling down ropes onto its bow, the balaclava-wearing hijackers made a daring - and seemingly well-rehearsed - raid of the oil tanker, as seen in alleged footage released by Fars news agency last night.

The wind was choppy, the skies overcast. With no navy escort, the Stena stood little chance.

Minutes later, at 4.19pm on Friday afternoon, the Stena Impero would “go dark” - not something normally done by commercial oil tankers in the Persian Gulf.

The first clue as it what happened was its abrupt change of course, which was picked up by marine tracking services. Its destination was a port in Saudi Arabia, but it had taken a sharp turn and was heading into Iranian waters.

Minutes earlier it had been boarded by Iranian Revolutionary Guards, who had hijacked the vessel using speedboats and a helicopter and turned off its communication systems.

Approximately 40 minutes later, a British-owned, Liberian-flagged ship Mesdar also went dark. The trackers picked it up following the same route as the Stena Impero. The crew onboard was questioned for an hour before the vessel was released, unlike the Stena which was escorted on to the coast of Bander Abbas in southern Iran.

British authorities were alerted back home and quickly called a meeting of Cobra to figure out their response.

The capture of one of their ships was something they had been dreading,though not something that had come entirely as a surprise.

Tensions have been heating up in recent weeks in the Strait of Hormuz, the world’s most important oil chokepoint.

At the start of the month, Gibraltar authorities - aided by a detachment of Royal Marines - detained a tanker which was suspected to be carrying Iranian oil destined for a refinery in Syria in breach of European Union sanctions.

"If Britain does not release the Iranian oil tanker, it is the authorities' duty to seize a British oil tanker," an Iranian official tweeted on July 5, the following next day, in response to the news. Revolutionary Guards issued similarly direct threats.

Fearing they would make good on them, the Navy sent Type-23 frigate HMS Montrose to shadow its tankers through the strait and dispatched another, HMS Duncan, for support. The Montrose sped to help Stena from Omani waters on Friday, but was an hour too late.

Jeremy Hunt, Foreign Secretary, had tried to defuse the situation last weekend by suggesting the UK was willing to release the supertanker, but a court in Gibraltar on Friday ruled to hold it for another 30 days.

The decision would have further angered Tehran, which has denied the oil was bound for Syria and accused the UK of acting in bad faith.

The legality of Britain’s impounding of the Grace 1 has been questioned, however sanctions lawyers say that as it had been travelling through British overseas territory it was subject to EU laws.

Revolutionary Guards yesterday tried to justify their seizure of the Stena with alternating claims, including that it had “violated maritime law”, had been driving on the wrong side of the water, risking an accident, and had in fact collided with an Iranian fishing boat whose distress call it ignored.

No such distress call was picked up by any other ship in the area.

Abbas Ali Kadkhodaei, spokesman of Iran's Guardian Council, which rarely comments on state matters, said they did not need an excuse to take the Stena and spelled out that it had been a tit-for-tat response.

"The rule of reciprocal action is well-known in international law and Iran's moves to confront the illegitimate economic war and seizure of oil tankers is an instance of this rule and is based on international rights," he said.

There is now something of a Mexican stand-off in the Gulf, with both countries seemingly unwilling to hand over the other’s ship.

“Iran has responded in a way that presents the UK with a problem,” Michael Stephens, Research Fellow at the Royal United Services Institute think-tank in London, told the Sunday Telegraph. “The ball is now in our court.

“The UK could choose to detain more Iranian ships, or look to gather a group of states around the table, such as France, Germany and the US, to see how, and in what ways, more pressure can be placed on Iran both economically and strategically,” he said.

However, he believed no major decision would be agreed on until Prime Minister Theresa May’s handover to Boris Johnson later this week.

The Foreign Office has stressed it is keeping separate the issues of Iranian threats in Gulf waters, EU sanctions policy on Syria, and the nuclear deal. But inevitably they have all become intertwined.

The latest Iranian aggressions can be tracked back to last year, when President Donald Trump pulled out of the 2015 nuclear accord and reimposed sanctions.

The Islamic Republic has legitimate frustrations over the American withdrawal to the deal - which it had been adhering to - that was supposed to swap limiting its nuclear programme for an end to sanctions crippling its economy.

At the same time as ratcheting up tensions, however, Mr Trump has made it clear he wants to avoid all-out war with Iran, as has the UK.

Mohammad Javad Zarif, Iran’s foreign minister, on Thursday offered an olive branch to Mr Trump - a deal which would see Tehran accept enhanced inspections of its nuclear programme in return for the permanent lifting of sanctions.

Mr Trump has sent Senator Rand Paul, rather than John Bolton, his hawkish anti-Iran national security adviser, for meetings with Mr Zarif, who is in New York on United Nations business. Neither has publicly responded to Mr Zarif's proposal.

However, hardliners and the Revolutionary Guard back home want out of the deal, saying the US’s pullout only proved what they always knew - that it cannot be trusted.

"I suspect Stena is a bargaining chip,” Charles Hollis, a former British diplomat in Iran, told the Telegraph. “It came only days after Zarif showed some willingness to open negotiations, which may have led some hardliners to want to disrupt things a little.

“I still don’t think any side is looking for a conflict,” said Mr Hollis, who is now managing director of risk management company Falanx Assynt. “The fact that there are some people on both sides were seeking a deescalation means there may be a deal to be found.”

He warned however, that Friday’s incident showed the margins for manoeuvre are “shrinking” and “the risks of unintended consequences growing.”