The recent assassination of top Irani general Qasem Soleimani has yet again started a strife between Iran and US. In response to the mounting tension, Irani president Hasan Rouhani has threatened to suspend the traffic of merchant vessels from the strait of Hormuz.  . The recent threat by Iranian president Hasan Rouhani led to a sudden rise in prices of crude oil by about $1 per barrel in just one day after the aforesaid threat.  Thus showing the vulnerability of world economy on the Strait of Hormuz. The Strait of Hormuz assumes a great importance as almost 20% of the total crude oil supplies passes through it. Closing of the strait would send the oil prices skyrocketing and would lead to catastrophic impact on the global economy. These recent happenings have again brought the law of maritime passage
Considering the importance of the strait of Hormuz, this article will seek to answer the question that whether Iran can block the Strait of Hormuz under International Law. This question will mainly be dealt in three sections. The first section, will discuss the legal claim of Iran over the strait of Hormuz. The second section will deal with the legal nature of the passage through the strait. Finally, the third section will deal with the necessity and proportionality of such measures.
TERRITORIAL CLAIM OVER THE STRAIT OF HORMUZ
The main treaty dealing with the maritime law is UNCLOS (United Nations Convention on Law of Sea, 1982) . According to UNCLOS, generally the territorial sea of a nation extends to 12 nautical miles . However, equidistant rule is applicable in order to delimit strait which lies between two opposite coastal nations. That is to say, no nation can extend its territorial sea beyond the equidistant line drawn through the middle of the strait  . This rule of delimitation has also been accepted by the ICJ as the Customary International Law in the matters of North Sea Continental Shelf Case.  This rule has also been reiterated in the Convention on Territorial Seas and Contiguous Zones, 1958. 
The strait of Hormuz measures 21 nautical miles and lies between Iran & Oman.  Though, Iran has not ratified the UNCLOS but it still has, legitimate territorial rights over the strait due to application of Customary International Law which also recognizes the equidistant rule.
RIGHT OF PASSAGE OVER THE STRAIT OF HORMUZ
While it is settled that around half strait of Hormuz does lie within the exclusive territorial seas of Iran, but it does not give Iran the freewheeling power to block the strait. According to the international law, merchant ships are granted the right to traverse through the territorial sea of a coastal nation.
These rights of passage are mainly right of innocent passage and the right of transit passage. Both of these rights impose different levels of duties on the coastal nations to provide unimpeded passage to the merchant ships.
Right of Innocent Passage
The right of Innocent passage is accepted as Customary International Law by ICJ, in the matters of Corfu Channel Case . This right has also been mentioned in the UNCLOS and the Convention on Territorial Seas and Contiguous Zones. 
As the name suggests, the right of Innocent passage provides, merchant ships the right to traverse through the territorial seas of any coastal nation as long as their passage does not prejudice peace, good order or security of the coastal State. Hence the right is suspendable in nature, such that a coastal nation can suspend this right if it believes the passage to be prejudicial to its security. 
However, since this right is entrenched in the Customary International Law, this right is not limited only to the state parties to UNCLOS.
Right of Transit Passage
Unlike the right to innocent passage, this right is only mentioned in treaties such as UNCLOS  and Convention on Territorial Sea and Contiguous Zones  however the state practice and opinion juris fails to confirm the right of transit passage as a Customary International Law.
The right of transit passage applies only territorial seas which provide an exclusive link between two larger bodies of waters unlike the right of innocent passage which applies to all the territorial waters.
However, this right provides greater security to merchant ships as this right is non-suspendable in nature.  This right is made absolute in nature as these straits are sensitive chokepoints in world economy and their freedom needs to be protected at all costs.
Right of Passage over the Strait of Hormuz
Now in literal sense, the Strait of Hormuz forms a strait which links the Persian Gulf’s EEZ to the Strait of Oman’s EEZ, as well as the high seas beyond thus it should be subject to the non-suspendable right of transit passage.
However, Iran has not ratified the UNCLOS and hence its powers can only be subjected to Customary International Law which does not recognize the right of transit passage. On the other hand, Oman has also expressed reservations to the right of transit passage while ratifying the UNCLOS. Thus, the Strait of Hormuz is not subjected to the right of transit passage but only to the right of innocent passage which is entrenched in the Customary International Law.
Since the merchant ships are only given the right to innocent passage, the strait of Hormuz can be subjected blockade if Iran considers the passage to be prejudicial to its security.
Though it is settled that Iran can suspend the right to innocent passage, but this power is not available to its whims and fancies. The right to suspend an innocent passage is limited to situations in which the passage poses a danger to its security, sovereignty or its territorial integrity. The blocking of strait by Iran, using its naval warfare will definitely amount to a “use of force” which has to pass the tests of necessity and proportionality.
As far as necessity is concerned, a nation can invoke the justification of self-defence under the Article 51 of the UN Charter if it takes an action in response to an “armed attack”. Hence, Iran can also not claim the justification of self-defence because a few isolated standoffs with US cannot be said to be “armed attacks” as they do not rise to the gravity required in Article 51 of the UN Charter 
Shivankar Sukul is a 2nd Year Law Student at National Law University, Jodhpur. He can be reached at firstname.lastname@example.org