What Is a Strait? Narrow Water That Decides Everything
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A narrow channel connecting two larger bodies of water is a small feature on a map and an enormous one in practice, because it concentrates traffic that has no alternative route. A handful of such channels carry the majority of the world's seaborne oil and a substantial share of everything else, and the fact that a ship cannot go around them is why the same names appear in every discussion of trade, of naval strategy and of law of the sea.
The ones that matter
A small number of chokepoints carry traffic out of all proportion to their size:
- •The Strait of Hormuz, between Iran and Oman, roughly fifty kilometres wide at its narrowest with a shipping channel far narrower, carrying around a fifth of global petroleum liquids with no practical bypass for most Gulf exports
- •The Strait of Malacca, between Malaysia and Sumatra, the shortest route between the Indian and Pacific oceans, carrying a very large share of trade between East Asia and everywhere west of it, and narrowing to under three kilometres at the Singapore end
- •The Turkish Straits, the Bosphorus and Dardanelles, which are the only exit from the Black Sea and are under a mile wide at points, running through a city of fifteen million
- •The Strait of Gibraltar, the Mediterranean's only natural connection to the Atlantic, fourteen kilometres wide
- •Bab el-Mandeb, at the southern end of the Red Sea, which is the approach to the Suez Canal and whose disruption forces ships around Africa at a cost of roughly ten days
- •The Danish straits, the Taiwan Strait, the Sunda and Lombok straits, and the artificial equivalents at Suez and Panama, which are canals and function as chokepoints in the same way
The law
Because a strait narrower than twenty-four nautical miles falls entirely within the territorial waters of the states on either side, there is a potential conflict between coastal sovereignty and the world's need to pass. The Law of the Sea Convention resolves it with the regime of transit passage, which applies to straits used for international navigation and which guarantees ships and aircraft of all states continuous and expeditious passage that the coastal state may not suspend, a stronger right than the innocent passage applying in ordinary territorial waters, since submarines may transit submerged and aircraft may overfly. Coastal states may regulate safety, pollution and traffic separation schemes and may not obstruct. Some straits have their own older treaty regimes that survive, most notably the Turkish Straits, governed by the Montreux Convention of 1936, which gives Turkey control and sets specific limits on the tonnage and duration of warship transits by non-Black Sea states, a document that has become unexpectedly prominent again.
The physical oddities
Straits behave strangely as water bodies because a narrow gap constrains flow. Currents through them are frequently fast and can be tidal, reversing direction, with the Strait of Messina's whirlpools supplying the original of Scylla and Charybdis and the Saltstraumen in Norway producing the world's strongest tidal current. Where two connected seas differ in salinity or temperature, a strait carries a two-layer exchange, with denser water flowing one way along the bottom and lighter water the other along the surface, which is what happens at Gibraltar as salty Mediterranean water sinks out into the Atlantic while Atlantic surface water flows in, and which submarines have historically exploited to pass silently by riding a current with engines off. Sediment transport, ice, fog and shallow water make many of them navigationally difficult on top of being crowded, which is why the busiest have mandatory pilotage, traffic separation and vessel reporting systems.
Why they shape strategy
The concentration of traffic makes a strait an instrument of leverage, and the history of naval strategy is largely a history of controlling them. Alfred Thayer Mahan's influential analysis at the end of the nineteenth century identified control of chokepoints as the basis of sea power, and British imperial strategy was built explicitly on holding Gibraltar, Suez, Aden, Malacca and the Cape. The same logic drives modern concern: a state whose imports must pass a strait controlled by a rival is exposed, which is the reasoning behind Chinese discussion of a Malacca dilemma and behind pipeline projects designed to bypass Hormuz and the Turkish Straits. Disruption is expensive and rapid, as the grounding of a single ship in the Suez Canal for six days in 2021 demonstrated by halting a substantial share of world trade, and as attacks on shipping near Bab el-Mandeb have shown by diverting traffic around Africa. The alternatives are longer routes, pipelines, strategic reserves and, increasingly, northern sea routes opened by retreating ice, none of which removes the dependence quickly.
The takeaway
A strait concentrates traffic that has no alternative, which is why a handful of narrow channels including Hormuz, Malacca, Gibraltar, Bab el-Mandeb and the Turkish Straits carry an enormous share of world trade and energy. Because narrow straits fall within territorial waters, the law of the sea grants transit passage, a stronger and non-suspendable right that allows submerged submarines and overflight, with the Turkish Straits governed by a separate 1936 treaty. Fast reversing currents and two-layer exchange flows make several of them physically difficult as well as crowded.