President Hassan Rouhani’s apparent threat to block the Strait of Hormuz in retaliation for looming US sanctions on Iranian oil exports has made headlines around the world. The strait is a vital route through which one-third of sea-borne oil flows to the Arabian Sea. In 2016, it amounted to 18.5 million barrels per day — the highest volume for any maritime chokepoint in the world. While Rouhani never explicitly referred to the waterway by name, his language seems to be an implicit warning to the West — and especially the United States. During his tour of Switzerland and Austria in early July, Rouhani said, “The Americans say they want to reduce Iranian oil exports to zero. … It shows they have not thought about its consequences.” He then added, “It would be meaningless that Iran cannot export its oil while others in the region can. Do this if you can and see the consequences.”

This is not the first time that Iran has made such a threat.

During the 1980-88 Iran-Iraq War, senior Iranian officials repeatedly issued warnings about closing the strait. In 1983, founder of the Islamic Republic and Supreme Leader Ayatollah Ruhollah Khomeini stated, "I warn the regional states as well as countries which make use of oil … that Iran, exercising its utmost power, will oppose this aggression and is determined to block the Strait of Hormuz, thus obstructing the passage of even a single drop of petroleum from there, should such an aggression be actualized." Late former President Akbar Hashemi Rafsanjani said in 1983, “We will block the Strait of Hormuz when we cannot export oil.” The following year, Rafsanjani elaborated, “We never said we will simply close the Strait of Hormuz, nor do we wish to have it closed, because if it were to be closed it would harm us too and we will incur losses. Therefore, we have always emphasized that we will close the Strait of Hormuz if the Persian Gulf becomes unusable for us. And if the Persian Gulf becomes unusable for us, we will make the Persian Gulf unusable for others also."

More recently, amid the tightening of sanctions on Iran during the presidency of Mahmoud Ahmadinejad, Vice President Mohammad Reza Rahimi said in December 2011, “If they impose sanctions on Iran’s oil exports, then not one drop of oil will flow from the Strait of Hormuz.” Iran never carried out that threat, even though its oil was sanctioned by not only the United States but also the European Union.