Strait of Hormuz is a waterway between the Sultanate of Oman and the Islamic Republic of Iran. It connects the Persian Gulf with the Gulf of Oman, and at its narrowest point, its width is 21 miles. It carries 17.2 million barrels of oil per day. Assuming a barrel of oil costs 55 US Dollars today, it takes approximately 1 billion dollars of oil each day, breaking one-sixth of world oils. It also hosts Liquefied Natural Gas from Oman, United Arab Emirates, and Qatar, which accounts for 26% of the LNG’s globally.
The strait is very busy from oil tankers all around the world, and with something as relevant and as crucial to the world economy as oil comes the security claims. Most of the tankers move around with warships or escorts. US and UK have their warships situated in and around the strait, and in fact, the UK claims the Royal Navy to have stopped the Iranian Revolutionary Forces trying to seize their ship in the Persian Gulf on July 10, 2019. Before that, Iran had been accused of taken two oil tankers from Norway and Japan (Iranian government officially denied this) and shot down a US drone in June. In later July, Iran also accused of seizing a ship from UAE, and a day later, it took two oil tankers from the UK; it released one of the UK ships immediately, but the other, Stena Impero, was only released on September 27. In the meantime, tensions rose after the US claimed it shot down an Iran drone in late July as well, but Iranian officials denied this, claiming the US had shot down one of their drones.
Under international law, the law of straits is governed under UNCLOS which states that as long as the ships are moving through the straits without delay, refrain from threat and use of force to the neighboring state(s), they have the right to move freely (right to transit passage). In the case of Strait of Hormuz, they also have to comply with the rules dictated by IMO as well. Under these rules, ships have to move through Iran’s territorial waters, but it could be argued that boats have the right to innocent passage if they enter Iran’s territorial waters as well. Stena Impero’s owners claimed that they were moving through the high seas, but these claims are not believable since the narrowest point of the strait is 21 miles, which means it has to be in the 12-mile-long territorial waters of Oman or Iran either way.
Iran signed UNCLOS but did not ratify it. But under international law, if a state does not protest the usage of rights born by a treaty (similar to Turkey’s 6-mile territorial water claims), it becomes customary law, and states are bound by it. Therefore, Iran is bound by UNCLOS and can’t close the strait during peacetime. But Iran being bound by this agreement doesn’t mean they could be forced by ICJ to let the ships pass through since Iran has to accept ICJ’s jurisdiction in the first place to be sued. Thus, the only thing left for states is to solve the situation without furthering the tensions diplomatically.