Contested-corridor topology, and how maritime chokepoints compose.

§0 · The topology, not the instance

The topology, not the instance.

Two contested corridors are currently active. The Strait of Hormuz has been closed to normal transit since 4 March 2026 under the Iranian Persian Gulf Strait Authority claim, at Day 195+ as of this composition. The Bab al-Mandeb, twenty-eight hundred miles to the west, entered the same class on 20 July 2026 when the Houthi movement declared and began enforcing a ban on Saudi port traffic. What was one contested waterway is now two, and the second was produced nine days after the collapse of the political mechanism at the first.

The useful question is not which corridor is worse, or how long either will hold. It is how contested corridors compose, couple, and resolve. The current situation is one instance of a mechanism that has produced contested corridors before, and one that carries specific consequences for how contract structure, freight hedging, and insurance premium composition should compose against maritime chokepoint risk going forward.

Read at the class rather than the instance, three things get named. The components that make a corridor contested. The mechanism that extends contestation from one corridor to a second. The conditions that produce resolution. The precedent that resolved contested-corridor governance most durably ran through thirteen years and required a specific set of conditions to assemble. Those conditions are not currently visible. What is visible is the mechanism that extends the class.

§1 · The components

A contested corridor has three components.

The first is a maritime chokepoint where a substantial share of a class of goods must pass, and where alternative routes exist but at materially higher cost. The Hormuz strait carries approximately 20 percent of global oil transit and approximately 25 percent of global LNG transit at normal state. The Bab al-Mandeb carries approximately 12 percent of global maritime trade by volume including approximately 30 percent of Suez-routed traffic. Both have alternatives. Both alternatives carry a cost delta that transmits through the delivered price of what moves through them.

The second component is an enforcement actor with an operational claim on the chokepoint. Enforcement can be state-formal, sub-state, or insurgent. The state-formal variant carries an assertion of legal authority: Iran's PGSA claim asserts permit, fee, and routing control over all Hormuz transits under legal argument that the international shipping associations have contested as a violation of international law. The sub-state variant carries an operational claim without a legal assertion: the Houthi movement asserts a ban on Saudi port traffic backed by attack cadence, not by legal argument. The distinction matters at the resolution surface. State-formal claims can be settled through diplomatic mechanism. Sub-state claims cannot.

The third component is enforcement cadence at a level that transmits through cost. Attack frequency, permit denial rates, transit slowdown, and war-risk insurance premium movements all carry the transmission. The threshold that turns a chokepoint from "at risk" to "contested" is the cadence at which enforcement becomes priced into the delivered cost of goods that route through it, not the frequency of specific incidents. IMF PortWatch recorded 34 Hormuz transits daily on 5 July 2026 against an 88-vessel baseline, with the Islamabad Memorandum still nominally in force. That is the transmission threshold. Not the collapse of the deal. The transit floor holding at 39 percent of baseline through the interlude itself.

The current topology carries two live instances at that transmission threshold. Hormuz has been at contested state since 4 March 2026, with the March activation and July collapse cycles now on record. Bab al-Mandeb entered contested state on 20 July 2026. Both are running at cadence that transmits through freight, insurance, and product cost. The topology also carries a third instance at watch. The Turkish Straits, where the Ukraine-Russia dynamic has produced attack-cadence pressure without yet reaching transit-floor transmission. Malacca and the South China Sea sit at watch but not at threshold; the enforcement cadence there is naval posture, not transit disruption.

The distinction between watch and live matters for how the topology reads forward. A chokepoint at watch can move into contested state through enforcement cadence rising to threshold. The mechanism of that movement is what the July 2026 extension of contestation from Hormuz to Bab al-Mandeb makes concrete.

§2 · The extension mechanism

Different actors. Same mechanism.

The Islamabad Memorandum was signed on 17 June 2026. It committed to a phased de-escalation of the Hormuz situation with corridor-governance provisions written into the framing. IRGC corridor enforcement continued through the interlude. On 8 July, IRGC struck three tankers in the Persian Gulf. The US responded with more than three hundred strikes across three rounds of attacks in five days. On 7 July, the Trump administration formally declared the deal void.

Twelve days later, on 20 July, the Houthi movement declared a ban on Saudi port traffic effective at 12:00 UTC. Vessels with Saudi-Arabian links have been attacked since. What the Houthis targeted specifically was Yanbu, the Red Sea port to which Saudi Arabia had pivoted crude exports as its Hormuz workaround. Aramco had run approximately 5 million barrels per day through Yanbu since March 2026, using the East-West Petroline connecting Gulf oilfields to the Red Sea coast. That flow was the largest active workaround the Hormuz closure had produced. Its capacity was Saudi Arabia's practical answer to the corridor problem.

The Houthi enforcement was aimed at that answer.

Iran did not coordinate this. The Houthi decision was made by the Houthi command with Houthi strategic reasoning. What produced the coupling was strategic alignment rather than tactical coordination. Iran benefits from Saudi crude flows being constrained regardless of which corridor carries them. The Houthis benefit from a broader confrontation across the region that keeps their strategic position elevated. Both actors had reason to pressure the Saudi workaround, and neither needed the other's permission to do so. What was one enforcement regime became two, and the two share a target without sharing a command structure.

This is the mechanism that generalises. A contested corridor extends when three conditions hold simultaneously. A second enforcement actor sits in geographic position to affect the workaround infrastructure the first corridor's closure has produced. That actor holds strategic interest aligned with the first corridor's contestation, meaning success at contestation delivers value to their independent strategic position. And cost transmission from the first corridor has already elevated freight and insurance premiums enough that the second contestation registers at the transmission threshold from its first incident, rather than requiring months of cadence build-up.

All three conditions held on 20 July 2026. The Houthi geographic position at Bab al-Mandeb sat directly across the Red Sea maritime axis from Yanbu. Houthi strategic interest in constraining Saudi capacity aligned with the Iranian PGSA regime's effect on Gulf flows. Freight and insurance premiums were already elevated on Persian Gulf traffic, so a Bab al-Mandeb war-risk overlay layered on top of an existing premium rather than reset a baseline. The extension arithmetic produced the second contested corridor in a single move.

Historical instances of contested-corridor extension carry the same three conditions. The Atlantic-to-Pacific extension of the Second World War maritime conflict produced two contested corridors linked by shared adversary rather than by tactical coordination. Japan and Germany did not coordinate their submarine campaigns; the strategic alignment of degrading Allied shipping capacity was sufficient for the effect to compound. The extension began within months of the Pacific war opening on 7 December 1941, because the shipping capacity that had been produced to compensate for the Atlantic losses was the target the Pacific campaign sat in geographic position to affect. Different actors. Same mechanism.

The First World War produced the same shape at Mediterranean-to-Atlantic extension. The German U-boat campaign that began as a North Sea and Atlantic contestation extended to the Mediterranean in April 1915 through Austro-Hungarian and German submarine forces operating from Pola and Cattaro. Coordination was minimal. Strategic alignment with the broader effort to constrain Allied shipping was total. Cost transmission had already occurred through the North Sea contestation.

The three-condition test is the extension arithmetic the topology carries. Where it holds, extension is not a scenario. It is the mechanism.

§3 · The coupling

The coupling, structural rather than tactical.

The coupling between Hormuz and Bab al-Mandeb is structural, not tactical. That distinction matters at the resolution surface, because structural couplings do not resolve when the actors are addressed separately. Only shared-interest resolution decouples them.

Structural coupling means the two enforcement regimes share a strategic interest that neither actor's individual resolution addresses. The Iranian PGSA claim contests Gulf outflow. The Houthi Saudi-port ban contests Gulf outflow through its Red Sea workaround. Both actors would need to accept a settlement in which Gulf outflow returns to pre-contestation cost structure. Iran has no motivation to accept that outcome unilaterally while Israeli hostility, US pressure, and regional non-recognition of PGSA all continue to hold. The Houthis have no motivation to accept it while Saudi participation in the Yemen conflict, Iranian material support, and their broader strategic elevation from Gulf-region attention all continue to hold.

Tactical coupling would look different. If Iran directed the Houthi Yanbu ban, the coupling would resolve when Iranian direction ceased. The evidence does not support that read. The Houthi decision followed Houthi strategic reasoning about the value of Red Sea contestation to Houthi position, which happens to align with Iranian interest rather than derive from Iranian instruction. This is the harder version of the coupling problem, because it cannot be untangled by cutting a single chain of command.

The test for coupling durability is direct. If either corridor resolved independently, what happens to the other. If Hormuz resolved through a Path A political mechanism with operational corridor governance signed by Iran, Saudi Arabia, the US, and mediating parties, the Bab al-Mandeb regime would persist. The Houthi strategic interest in constraining Saudi capacity does not derive from the Hormuz situation. It runs on its own reasoning, and the Yemen conflict, the Iran-Houthi material relationship, and the broader Red Sea contestation dynamics all sustain it independently. Saudi crude routing to European and Asian refiners would still carry a Bab al-Mandeb premium after a Hormuz settlement. The freight and insurance cost floor would move down from its current elevation, but not to pre-contestation baseline.

The reverse case runs the same way. If Bab al-Mandeb resolved through a Houthi-Saudi accommodation independent of Iran, the Hormuz PGSA regime would persist. Iranian strategic interest in Hormuz contestation does not derive from Houthi enforcement. The two regimes hold each other in the mutual position not because either actor is reinforcing the other tactically, but because both actors are executing individually rational strategies whose effects compound into the coupled outcome.

That produces what the resolution surface calls the partial-resolution problem. Solving one corridor without simultaneously solving the coupled corridor does not eliminate the topology. It transfers the contestation weight to the un-resolved corridor. The freight capacity, insurance capacity, and shipping fleet that were routing around the resolved corridor now concentrate against the remaining one, elevating the cost floor there and creating attention shift that can attract new enforcement cadence. Historical instances show this pattern. When the Barbary Coast contestation partially resolved through the 1805 Treaty of Tripoli, corsair activity concentrated on the remaining un-treatied ports, and the eventual full resolution required the 1815 Second Barbary War specifically because the partial resolution did not decouple the class.

Structural coupling therefore requires shared-interest resolution or coincident resolution across both corridors. Neither is quick to assemble in the current configuration of interests, because the conditions the next section walks through have taken decades to produce in the historical instances that succeeded.

§4 · Precedents

Montreux, and what it required.

The Turkish Straits Convention signed at Montreux on 20 July 1936 is the historical precedent for contested-corridor resolution that held durably. The Convention governed the Dardanelles and Bosphorus, connecting the Aegean and Black Seas, and it produced ninety years of operationally settled transit governance. It has not been reopened. Turkish sovereignty over the Straits under specific international obligations has held through the Second World War, the Cold War, the collapse of the Soviet Union, and the current Russia-Ukraine dynamic. What Montreux delivered is exceptional. What Montreux required is instructive.

The Lausanne Straits Convention of 24 July 1923 was the initial attempt at settlement following the First World War. It attempted to internationalise the Straits through a demilitarised regime supervised by a Straits Commission with representation from major powers. It failed operationally within a decade. Turkey rejected the demilitarised provisions as violating its sovereignty. The League of Nations mechanism intended to enforce the arrangement proved to be non-enforcing. Soviet and Balkan states signed with reservations that limited the framework's practical reach. By the mid-1930s the Straits Commission had become inactive and Lausanne had lost its enforcement architecture. Turkey formally requested renegotiation on 10 April 1936.

The renegotiation took approximately three months of concentrated diplomacy at Montreux, but the conditions that made those three months productive had taken thirteen years to assemble. Four conditions specifically had to be in place.

Exhaustion of alternatives.

The Lausanne regime had been demonstrated to be unworkable. The parties had run the alternative arrangement for thirteen years and observed that the internationalised regime did not produce the governance function it was designed to deliver. A new settlement could be composed only after the failure of the old one had been observed at operational scale, not asserted from principle.

Viable international legal structure.

The Montreux Convention was composed against a specific legal architecture that assigned Turkish sovereignty over Straits governance subject to defined international obligations. The distinction between "internationalised" (Lausanne) and "sovereignty-with-obligations" (Montreux) was legally material. It produced a structure where the party with strongest interest in operational functioning of the Straits, Turkey, held the authority to enforce the arrangement. Sovereignty was not neutralised. It was structured.

Buy-in from enforcement-capable parties.

Turkey, the Soviet Union, the United Kingdom, France, Bulgaria, Romania, Greece, and Yugoslavia all signed. Japan signed with reservations. Italy did not sign until 1938, but the pattern of buy-in across enforcement-capable parties was substantial enough that Montreux held. The parties that could have unilaterally contested the Straits regime chose to accept the settlement instead. That acceptance was not sought; it was demonstrated through prior contestation and eventual convergence around an arrangement that served each party's interest sufficiently to make sustained contestation unattractive.

A specific international environment.

The mid-1930s carried a rising anticipation of European conflict. The Great Powers accepted Montreux in part because they preferred a stable Straits regime under Turkish authority to an uncertain regime under any other arrangement, particularly as the Nazi and Soviet postures made contested-corridor governance a strategic liability. The environment produced an acceptance level that principle could not.

Applied to the current situation, the four conditions read as follows. Exhaustion of alternatives is at approximately month seven, with three activation-and-collapse cycles observed in 2026: March activation, June-July Islamabad-and-collapse, August re-escalation pressure. Viable international legal structure does not yet exist; the PGSA claim is legally contested and no counter-arrangement with defined operational authority has been proposed. Buy-in from enforcement-capable parties is not present; Iran, the US, Saudi Arabia, Israel, the UAE, and the Houthis have not converged on any settlement. The international environment is one where contested-corridor resolution carries lower immediate priority than the underlying conflicts driving contestation, specifically the Israel-Iran hostility, the Yemen conflict, and the US-Iran adversarial dynamic.

The gap between month seven and year thirteen is not a criticism of the current effort. It is the timeline the historical precedent produced. Lausanne to Montreux ran through failure at the internationalised arrangement, sustained observation of that failure, Turkish diplomatic re-engagement, and eventual convergence around a sovereignty-with-obligations structure that all enforcement-capable parties accepted. The condition set that made Montreux possible in three months of active negotiation was assembled through thirteen years of prior failure and adjustment.

Other historical instances of contested-corridor resolution reinforce the timeline. The Barbary Coast contestation ran from approximately 1785 to 1815, thirty years in which contested-corridor arrangements were tried repeatedly with tribute payments, treaties, and partial military engagements before the 1815 Second Barbary War produced settlement. The Suez Canal contestation of 1956 produced a settlement through UN mechanism within months, but only because British, French, and Israeli military action had produced immediate international pressure sufficient to force resolution against active occupation. That is not the current situation. The Malacca Strait, at watch through the 1990s and 2000s as piracy and sovereignty tensions rose, resolved through multilateral cooperation between Indonesia, Malaysia, and Singapore over approximately fifteen years of coordinated patrol architecture. Malacca is instructive because it shows non-contestation resolution when strategic alignment does not exist among the actors capable of contestation. That is also not the current situation.

The forward implication is direct. The topology carries a resolution mechanism at the historical precedent scale of years and decades, not months. What accelerates it is the specific set of conditions the next section walks through.

§6 · The extension horizon

The extension horizon, and what a third corridor produces.

The mechanism §2 walks makes third-corridor extension a specific set of watch conditions, not a general concern about geopolitical risk. The three watchlist corridors carry different transmission arithmetic.

Turkish Straits.

The Turkish Straits carry approximately 3 percent of global oil transit but approximately 25 percent of Black Sea maritime trade including the Ukrainian and Russian grain exports that anchor global agricultural supply. TurkStream carries approximately 15 billion cubic metres per year of gas to European buyers including Hungary and Serbia. Attack-cadence pressure has been visible through the Ukraine-Russia dynamic since 2022 without reaching transit-floor transmission. The extension arithmetic requires an enforcement actor with strategic alignment, geographic position, and cost-transmission acceleration. Direct Russian military action against Turkish Straits transit would produce all three simultaneously and would arrive as a step change rather than as cadence build-up. The transmission at that step would elevate European gas prices by an additional 15 to 20 percent above current TTF elevation, spike Ukrainian and Russian grain export prices with immediate downstream effect on North African and Middle Eastern food inflation, and add a third war-risk premium layer to freight rates already carrying Hormuz and Bab al-Mandeb overlays. The horizon on which this becomes the higher-probability scenario is set by the Ukraine war trajectory. A Russian escalation cycle that includes maritime pressure on Turkish shipping would produce the third live corridor within weeks of the escalation choice.

Malacca Strait.

The Malacca Strait carries approximately 25 percent of global oil transit and approximately 30 percent of global maritime trade by volume. It has been at watch through the 1990s and 2000s with piracy and sovereignty tensions that resolved through Indonesia-Malaysia-Singapore multilateral cooperation. It sits at watch again through the current US-China dynamic. What would move it to live is a shift in enforcement cadence by a state actor rather than by insurgent or piracy actors. Chinese naval posture in the South China Sea has intensified through 2025 and 2026 in relation to Taiwan, the Philippines, and Vietnamese exclusive economic zone claims. None of that posture currently produces Malacca transit-floor transmission. What would produce it is a Taiwan Strait crisis in which Chinese enforcement of transit conditions became operational rather than declaratory. The horizon on that is not a market forecast. It is a strategic scenario that world energy markets, freight markets, and Asian economies must carry as background risk because the transmission at Malacca contestation would exceed all current corridor contestations combined. The insurance industry has been quietly building capital reserves against Malacca scenarios since 2024. That the build is quiet rather than public reflects the register at which it must occur.

European sub-corridor.

The European sub-corridor watchlist runs on different arithmetic. It is not a chokepoint contestation; it is an infrastructure sabotage scenario. Nord Stream was destroyed in September 2022. Subsea cable cuts affecting European and Baltic connectivity have occurred repeatedly since 2023 with attribution ranging from Russian intelligence services to unclear actors operating through commercial vessels. The specific mechanism at watch is Russian hybrid pressure on European gas import routes and undersea cable networks, timed to leverage moments of European political vulnerability. The extension arithmetic here requires enforcement cadence that transmits through cost, and the transmission runs through insurance markets on subsea infrastructure and through gas price volatility on any successful pipeline disruption. The forward horizon is short. Cable and pipeline incidents at cadence sufficient to price into commercial insurance are the transmission threshold, and the industry is already moving.

The transmission arithmetic.

The specific dollar and euro figures that a third corridor produces are transmission-chain-dependent. Turkish Straits contestation adds an estimated 40 to 60 dollars per barrel to Russian and Kazakh crude routing through Novorossiysk and the CPC terminal, elevates European gas by 15 to 20 percent above current TTF, and adds an estimated 800 to 1,200 dollars per FEU to Mediterranean and Black Sea containerised freight. Malacca contestation at even partial cadence produces catastrophic step changes: oil markets have not traded a 25 percent transit disruption at Malacca since the strait became the dominant Asian oil chokepoint, and the modelling capacity to price it is limited. The insurance industry's Malacca reserves are calibrated to loss scenarios in the hundreds of billions of dollars for the shipping and cargo classes exposed. European sub-corridor scenarios run in the tens of billions per year in gas market and subsea infrastructure exposure.

The aggregate reading of the extension horizon carries a specific structural implication for global markets. Businesses, insurers, and institutional investors currently composing risk models against a two-corridor topology are composing against the current state, not the forward state. The mechanism that produced the second corridor within nine days of the first political-mechanism collapse is the mechanism that produces the third. What moves the horizon is any escalation cycle in Ukraine, the Taiwan Strait, or Iran that carries a maritime pressure component. The cost of assuming that current corridor count is the ceiling is the same class of assumption that businesses composing against 2025 baseline conditions carry: it is a plan against a configuration the underlying reality is already leaving.