The US blockade of the Strait of Hormuz aims to cut off Iran's oil exports and punish any ship that pays a toll for transiting the waterway. somkanae sawatdinak/ShutterstockA US-sanctioned tanker with links to China, the Rich Starry, has transited the Strait of Hormuz, despite the US blockade of the waterway. According to the respected maritime news and intelligence agency Lloydslist, the Rich Starry is falsely registered in Malawi, but is Chinese owned and carrying a Chinese crew. It is subject to US sanctions for carrying Iranian goods. It is not known what the vessel is currently transporting.Having been anchored off the UAE, the Rich Starry is not technically in breach of the blockade, but the incident has raised fears of a potential confrontation between the US and China in the region. Other vessels are reported to be waiting to transit the Strait, despite the US blockade.The decision to impose a blockade on Iranian ports in the vicinity of the Strait was announced by the US president, Donald Trump, following the breakdown of US-Iran peace talks in Islamabad on April 11. Trump’s announcement was clarified by a statement on April 12 from US Central Command, which stipulated that the operation would prevent ships entering and exiting Iranian ports and coastal areas while not impeding vessels transiting the Strait to and from non‑Iranian ports. Trump also announced that: “I have also instructed our Navy to seek and interdict every vessel in International Waters that has paid a toll to Iran. No one who pays an illegal toll will have safe passage on the high seas.” It remains unclear as to whether this will be implemented.The Strait of Hormuz has been as good as closed since shortly after the US and Israel launched their attacks on Iran at the end of February. Most ship owners, charterers and insurers are unwilling to accept the financial risk – and risk to human life – that transiting the Strait under threat of Iranian attack would entail. Blockades are used to convert naval dominance into advantage on land by preventing imports and exports of goods, in Iran’s case oil, to put pressure on an adversary’s population and government by hurting their economy. Likewise, Iran’s strategy of closing down the Strait after it was attacked intended to disrupt the global economy in order to put international pressure on the Trump administration.Iran has long threatened to use its geographical proximity to the Strait of Hormuz to close it down. Having demonstrated how effective this can be in disrupting oil and liquid natural gas prices, Tehran has been flexing its muscles by demanding that ships wanting to transit the waterway pay a tariff of up to US$2 million (£1.5 million). Lloydslist reported on March 25 that “a total of 26 vessel transits through the strait have followed a route pre-approved under an IRGC [Islamic Republican Guard Corps] ‘toll booth’ system that requires the ship operators to submit to a vetting scheme”.This was reportedly a sticking point in negotiations between the US and Iran in Pakistan on April 11. Tehran wants to retain control of the Strait and the ability to levy tolls from transiting ships. The US is demanding that the maritime right of free passage must be enforced. It was when the first round of talks ended in deadlock that the US president decided to impose the naval blockade.Former US diplomat to the Middle East, David Satterfield, told the BBC on April 13 that it was now about which country could absorb more pain, adding: “The Iranians believe … that they can absorb more pain for a longer period than their opponents can.”Expensive – and risky – gambitThe cost calculus is asymmetric. It will be more expensive for the US to maintain its blockade than it was for Iran to close the Strait. The question will be whether Washington can sustain interdiction long enough to effectively undermine the regime – always remembering that the Islamic Republic has potentially had decades to prepare for this sort of scenario.If the blockade can be implemented effectively, it could – in time – have an effect on an economy wrecked by years of sanctions and further weakened by the recent war and nationwide protests in January. The question is how long that might take.To be effective, the blockade will require considerable naval resources. The US is reported to have as many as 21 warships in the Middle East, including the aircraft carrier USS Abraham Lincoln and the USS Tripoli, an amphibious assault ship with a complement of marines who are trained to board ships using helicopters and small boarding craft.This introduces another layer of risk as assets operating near to the Iranian coasts will need to be protected against Iranian missiles, drones and fast attack craft. So, this would be resource‑intensive, operationally demanding and thus politically exposed for the US.How the US will go about enforcement remains to be seen. In December and January, US naval and coastguard ships boarded and seized several vessels linked to Venezuela’s shadow fleet that had broken America’s blockade. Whether it would pursue the same action with a vessel linked to China is another matter though. And while another option would be to fire warning shots, these can be dangerous around tankers because of the risk of oil spillage, as well as the obvious political risk attached to Chinese-linked vessels. US coastguard boards the Marinera (footage suppied). It’s not clear at present that imposing a blockade on the Strait of Hormuz will restore free navigation of the waterway any time soon. But it now appears that, in the absence of free navigation, some countries have decided to call America’s bluff and attempt to transit the waterway in defiance of the US blockade. And the big concern must be the serious risk of escalation if the US attempts to enforce the blockade on a Chinese-owned vessel. None of this will be welcomed by the US president and his national security team.Basil Germond does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.