"If Britain does not release the Iranian oil tanker, it is the authorities' duty to seize a British oil tanker," Rezaei wrote on Twitter. He also said that Iran would respond "without hesitation" to "bullying" from other countries.

Crown Copyright Royal Marines on the deck of Grace 1 on July 4, 2019.

Iran has already summoned the U.K. ambassador in Tehran to lodge a formal protest about the seizure of Grace 1, which Iranian authorities have branded as illegal and a "form of piracy." British authorities have, not surprisingly, dismissed this characterization of the situation. Approximately 30 Marines from 42 Commando, Royal Marines, assisted in the operation to take control of the tanker on July 4, 2019. A number of them rappeled onto the ship's deck from a Royal Navy AgustaWestland AW159 Wildcat helicopter, while others, along with additional security forces personnel from Gibraltar, arrived on small boats. The War Zone just recently published an in-depth feature covering how these types of boarding operations get planned and executed, which you can read here.

The exact chain of events that led to the mission remains curiously murky. British authorities have said they suspect Grace 1 of carrying Iranian oil bound for the Baniyas Refinery in Syria's port city of Tartus. Baniyas is under European Union sanctions over Syrian dictator Bashar Al Assad and his government's actions during the country's brutal civil war. However, Spain, which disputes British control over Gibraltar and does not recognize the waters around the territory as being under the control of the United Kingdom, has claimed that U.K. personnel seized the tanker at the request of the United States. The U.S. government has its own sanctions in place targeting all exports of Iranian oil to any destination, which has been a serious blow to Iran as oil sales are a major source of the country's income. Spanish authorities say they are preparing to make their formal complaint about the incident, but over violation of what they say is their sovereign territory.

Crown Copyright Boats belonging British authorities in Gibraltar head out to support the seizure of the Grace 1 on July 4, 2019.

There are also reports that U.S. government did provide intelligence about the Grace 1 to the British to enable to seizure. On July 4, 2019, U.S. National Security Adviser John Bolton sent a tweet praising the operation, but also corroborated the British position that the mission was in response to E.U. rather than American sanctions. "Excellent news: UK has detained the supertanker Grace 1 laden with Iranian oil bound for Syria in violation of EU sanctions," he wrote. "America & our allies will continue to prevent regimes in Tehran & Damascus from profiting off this illicit trade."

It's not clear why the Iranian tanker was even sailing past Gibraltar in the first place, as it would have been a much shorter route to sail to Syria from Iran by way of the Suez Canal. Some have also suggested that this circuitous route might have been supposed to help the tanker escape interdiction, but its not clear why this would have been the case. It seems much more likely that this had to do with reports in April 2019 that Egypt, which controls the Suez, had begun blocking access to Iranian tankers headed for Syrian ports. Grace 1's general legal status, by itself, is another issue. The ship still carries indicating that it is registered in Panama, despite Panamanian authorities saying they delisted it in May 2019 after the Panamanian National Security Council issued a notice saying that it "may be participating in terrorism financing, in support of the destabilizing activities of some regions led by terrorist groups."

Crown Copyright A handout photo the U.K. Ministry of Defense released showing Grace 1 during the boarding and seizure operation. The ship is still marked as being flagged in Panama, despite this not having been the case since May.

Whether or not Iran decides to follow through on Rezaei's tit-for-tat threat remains to be seen. It is certainly true that Iran, and the IRGC especially, routinely makes grandiose threats, or otherwise highlights its ability to do so, without following through. At the same time, Rezaei himself has a very notable history of speaking his mind, including reportedly telling then-Iranian Supreme Leader Ruhollah Khomeini in 1988 that Iran could not win the war against Iraq. In 1997, he was formally ejected from the IRGC, purportedly, in part, over his indifference to perceived threats of a potential American attack. In 2009, he criticized President Mahmoud Ahmadinejad for questioning the Holocaust, saying the comments were "not useful" and threatened future rapprochement with the U.S. government. But it would also not be the first time Iran has seized a foreign commercial ship or directly retaliated against the United Kingdom. In 2015, the IRGC briefly took control of the Danish-owned container ship Maersk Tigris, which is flagged in the Marshall Islands, over a legal dispute. Just on July 4, the Maersk Line announced it would be adding a "war risk" surcharge to any shipments passing though the Persian Gulf owing to current regional tensions.

In 2007, the IRGC also detained 15 members of the British Royal Navy in the Persian Gulf for nearly two weeks. The sailors, then assigned to the Type 22 HMS Cornwall, had been searching a merchant ship at the time. Iran claimed that they were in Iranian waters, while the British said the operation had been in Iraqi waters. This incident was somewhat reminiscent of the IRGC's detention of a number of U.S. Navy sailors in the Persian Gulf in 2016. Iran is also increasingly in a dispute with the United Kingdom and other European countries over the fate of the controversial deal over its nuclear program. The United States pulled out of this agreement, known as the Joint Comprehensive Plan of Action (JCPOA), in 2018, leaving the remaining parties to try and salvage it. Iran recently accused the United Kingdom, France, and Germany of failing to uphold their obligations and now says that it will begin enriching uranium to levels above the JCPOA's limits, which threatens to upend the deal for good. The concern is that Iran growing its stockpiling of enriched uranium could reduce the time necessary to "break out" and build nuclear weapons should it decide to do so.