5-Minute Explainer — Straits
Why I’m making this
I’m putting together these short explainers for readers who follow geopolitics but often find themselves navigating a maze of jargon, acronyms, and assumptions. This series breaks things down in more a more digestible format.
What is a strait?
A strait is a naturally formed, narrow waterway connecting two larger bodies of water, situated between two landmasses. The defining quality is that it is a through-passage — you enter one body of water and exit into another.
There are distinguishing features between straits and other geographical features:
Canals are man-made. The Suez and Panama canals are built infrastructure, owned and operated by states, which can set access conditions and charge transit fees. Straits, conversely, are not built. They are geography. Canals also fall within the territorial sovereignty of states and are governed accordingly by a separate set of legal architectures, including specific agreements relating to the freedom of passage.
Channels are typically longer, wider and less constricted, presenting less of a bottleneck for passage.
Bays and gulfs are indentations into land, not through-passages. They do not open to other bodies of water.
Sounds are broader and typically run parallel to a coastline, separating a mainland from an island or connecting two larger seas. Less constricted than a strait, and rarely a chokepoint, and usually within the territorial sovereignty of a single state.
More than 200 straits worldwide qualify as international straits — connecting two areas of international waters and used for navigation. A handful define global trade and military access. Most do not.
Who is involved?
A strait is typically situated within the territorial waters of the bordering state or states, defined under international law as extending up to 12 nautical miles from the coastline.
Sovereignty over a strait may rest with a single state, when the strait is narrow enough that both banks fall within one country’s territorial sea. The Turkish Straits, which include the Bosphorus and Dardanelles, fall entirely within Turkish territory.
Or it may be shared between more than one state. The Strait of Hormuz borders Iran to the north and Oman to the south. The Strait of Malacca runs between Malaysia and Singapore on one side and Indonesia on the other.
The bordering state is sovereign. But that sovereignty is legally conditioned. The fact that a strait serves international navigation places it under a distinct legal regime that limits what the coastal state can do with it.
What governs straits?
The primary framework: United Nations Convention on the Law of the Sea (UNCLOS)
The main international instrument is UNCLOS, adopted in 1982 and in force since 1994. 172states are party to it. Part III of UNCLOS, Articles 34 to 45, establishes a specialized legal regime for straits used for international navigation.
Transit passage vs. innocent passage:
Innocent passage is the older concept, rooted in customary international law and first formally codified in the 1958 Geneva Convention on the Territorial Sea. It applies to a state’s territorial sea generally and gives ships the right to pass through, provided the transit is continuous, expeditious, and not prejudicial to the coastal state’s security. The coastal state can temporarily suspend innocent passage for security reasons. Submarines must surface and show their flag.
Transit passage was introduced by UNCLOS in 1982 specifically for international straits. All ships and aircraft — including warships and submerged submarines — have the right of continuous and expeditious transit. The coastal state cannot suspend it. Not for security reasons, not during armed conflict. Overflight of aircraft is also covered, which innocent passage does not provide.
The distinction exists because UNCLOS extended territorial seas from 3 to 12 nautical miles, which absorbed the open-water corridors that had previously run through the center of many straits. Without the transit passage regime, coastal states would have gained effective veto power over global navigation through their territorial waters. Transit passage was the compensating mechanism, designed to be deliberately more liberal than innocent passage because the geographic and economic stakes are higher.
Strait-specific agreements
Some straits operate under their own dedicated treaties, preserved by UNCLOS as carve-outs from the general framework. The clearest example is the Montreux Convention of 1936, which governs the Turkish Straits. Under Montreux, commercial vessels have free passage in peacetime, but Turkey controls warship transit with advance notification requirements, tonnage limits, and the authority to close the straits to warships of belligerent states in wartime. Turkey exercised this in 2022, closing the straits to warships of nations at war following Russia’s invasion of Ukraine. The convention held. No state challenged it.
Most significant straits have no equivalent bespoke arrangement. Hormuz, Malacca, Bab el-Mandeb, and the Taiwan Strait are all governed by the general UNCLOS framework (China, unlike the United States, ratified UNCLOS), or, where states are non-parties, by what customary international law requires.
What happens when states haven’t signed UNCLOS?
This is where the framework shows its limits, as evidenced in the case of the current crisis on Hormuz.
Iran signed but did not ratify the 1958 Geneva Convention on the Territorial Sea, and signed but did not ratify UNCLOS. It is not a formal party to either instrument. Its domestic maritime law of 1993 recognizes only innocent passage and makes no reference to Hormuz as an international strait subject to transit passage. The United States ratified the 1958 Geneva Convention but has never ratified UNCLOS, making it the only major maritime power outside the 1982 treaty.
Both states therefore stand in different legal positions relative to the governing frameworks, yet each invokes them in contradictory ways. The US position is that transit passage has crystallized into customary international law, binding on all states regardless of ratification. Iran’s position is that as a non-party to UNCLOS, only the weaker innocent passage regime applies at Hormuz. Both sides claim custom. They disagree on what custom requires.
The legal foundation predating this dispute is the Corfu Channel Case (International Court of Justice (ICJ), 1949), in which the International Court of Justice ruled that states cannot obstruct warship passage through international straits used for navigation between two parts of the high seas, even through their territorial waters. This case anchors the argument that freedom of passage in straits is a principle of customary international law, independent of UNCLOS entirely.
With that in mind however, Iran’s consistent and public rejection of transit passage, long predating and maintained throughout the current conflict, likely qualifies it as a persistent objector under customary international law, meaning the rule, even if binding on others, may not bind Iran under this framework. And this is where the dispute lies.
Why does it matter now?
The Strait of Hormuz Crisis has made this an immediate question rather than a theoretical one. It has brought the spotlight to the impact that closing a single strait can have on international markets and freedom of navigation.
Around 20 percent of global oil and significant volumes of liquefied natural gas transit through a corridor 21 nautical miles wide at its narrowest point. Disruption under Operation Epic Fury drove oil prices to levels not seen in years and prompted the IEA to describe the shock as the gravest to global energy supply since 1973.
Hormuz is the acute case, but the underlying vulnerability extends to other cases. Several other straits carry comparable systemic weight:
Strait of Malacca — approximately 80,000 vessels annually, carrying the bulk of energy imports to China, Japan, and South Korea. The shortest sea route between the Indian and Pacific Oceans.
Bab el-Mandeb — connecting the Red Sea to the Gulf of Aden. Already disrupted by Houthi attacks on commercial shipping since late 2023, demonstrating that the Hormuz pattern is not uniquely vulnerable.
Taiwan Strait — China does not accept it as an international strait subject to transit passage. The United States does. This divergence presents a potential future flashpoint between the two great powers.
Strengths of the Current System
A stronger protection than the alternative. Transit passage is non-suspendable, covers warships and aircraft, and requires no prior authorization. It is a materially stronger right than innocent passage, and a compensating mechanism for the extension of territorial waters from 3 to 12 nautical miles.
Anchored in custom, not only treaty. The Corfu Channel case grounds freedom of navigation in straits in customary international law, giving it legal force independent of UNCLOS ratification.
Broad acceptance. 172states are parties to UNCLOS, and most non-parties — including the United States — accept its transit passage provisions as reflective of established custom.
A standing judicial mechanism. UNCLOS established the International Tribunal for the Law of the Sea (ITLOS), a permanent judicial body empowered to issue binding rulings and provisional measures on law of the sea disputes. Unlike most areas of international law where no dedicated forum exists, states have a formal avenue to contest violations and obtain legally binding decisions.
Bespoke arrangements can hold. The Montreux Convention demonstrates that a strait-specific treaty with a capable and interested governing state can produce stable, respected outcomes even in wartime.
Fault Lines
Enforcement is unilateral and discretionary. UNCLOS has no institutional enforcement mechanism. While ITLOS and the ICJ can issue binding rulings, and have done so, including ordering Russia to release Ukrainian vessels seized in the Kerch Strait in 2019, they have no direct mechanism of enforcing the decisions. Russia ignored the order. The Corfu Channel case in 1949 produced a damages award against Albania that took 43 years to collect. Enforcement is a function of geopolitical will, after the legal process, and when the dominant power’s commitment is uncertain, the framework has no fallback.
The non-ratification problem. The framework’s authority over non-parties rests on the contested claim that transit passage has become binding customary law. That claim is legally defensible but not universally accepted, which means states can selectively reject treaty obligations without formally exiting any agreement.
The passage regime dispute. Coastal states with an interest in maximizing their own authority consistently assert that innocent passage, not transit passage, applies in their waters, and have done so systematically. During the UNCLOS III negotiations, a coalition of strait states including Iran, Oman, Indonesia, Malaysia, Morocco, and Spain collectively opposed the transit passage regime for precisely this reason. That position has persisted in state practice: Iran and Oman at Hormuz, Indonesia and Malaysia at Malacca, all assert greater authority over passage than the transit passage regime permits. This depresses the effective reach of the stronger protection across several of the world’s most critical waterways simultaneously.
Inconsistency across straits. The Montreux carve-out means different straits operate under materially different legal regimes. There is no uniform standard. The absence of a dedicated convention for most critical straits leaves the general UNCLOS framework as the only applicable instrument, with all its limitations intact.
Sovereignty tension is structural. The transit passage regime was designed to limit coastal state authority, a restriction bordering states that ratified the convention accepted as part of the UNCLOS bargain. States that rejected that bargain, or find the restriction increasingly costly, have a standing incentive to contest it. The framework depends on continued political acceptance of constraints on sovereignty that some states regard as inconvenient.
Successful violations erode the rule itself. International legal frameworks depend not only on formal compliance but on the perception that violations carry costs. Iran is actively attempting to replace the transit passage regime with a new legal architecture, including domestic legislation asserting permanent sovereignty over the strait and the imposition of transit fees. If that attempt is absorbed without effective reversal, it sets a precedent. Other coastal states with long-standing ambitions to assert greater authority over straits they border gain both a legal reference point and a political signal that the transit passage regime is negotiable. This could result in a cascading effect on international navigation.