Throughout history, sea power has been one of the most effective instruments not only for winning wars, but also for achieving political objectives without going to war. From the age of sail to the age of steam, and from dreadnoughts to aircraft carriers, powerful navies have often been able to alter the decisions of governments without employing their firepower.

The dictum attributed to the English republican Oliver Cromwell, “A warship is the best ambassador,” expresses this understanding with remarkable simplicity. This concept would reach its maturity within the British tradition of sea power after Nelson and become one of the principal instruments of the imperial foreign policy of the nineteenth century. In the twentieth century, the United States inherited and employed the same method on a global scale.

Gunboat diplomacy is the employment of sea power as an instrument of coercive diplomacy in times of crisis. It carries with it a clearly perceptible willingness to employ firepower if necessary. Throughout history, gunboat diplomacy has therefore remained one of the most flexible instruments available to states in the conduct of foreign and security policy.

Naval forces possess a unique combination of flexibility, endurance, sustainable firepower, self-sufficiency and mobility. They can operate freely on the high seas and, unlike land and air forces, their deployment to a crisis area does not normally depend upon obtaining transit permission from third countries. These characteristics make naval forces among the most effective and flexible instruments that can be employed from the very outset of a crisis.

No permission is required for deployment or passage on the high seas. Through the regimes of innocent passage and transit passage, naval forces may also navigate through foreign territorial seas, international straits and relevant waterways in accordance with international law. Warships can emerge from beyond the horizon and, through their physical presence and imposing appearance alone, generate a profound psychological effect upon both governments and public opinion. Once their mission has been accomplished, they can rapidly withdraw beyond the horizon and disappear from public view. Yet even when they remain over the horizon, the knowledge of their presence can continue to influence the psychology and calculations of governments and societies.

Gunboat diplomacy, generated by both the tangible and intangible presence of a warship, involves the limited employment—or threat of employment—of naval forces short of war, with the purpose of bringing an international or domestic dispute to a conclusion favorable to one’s own state, or of preventing, limiting or delaying the material and political consequences that might otherwise arise from such a dispute.

The modern theoretical framework of this method was developed by the British diplomat and naval strategist Sir James Cable. In his 1971 work Gunboat Diplomacy: Political Applications of Limited Naval Force, Cable defined gunboat diplomacy as the use, or threat of use, of limited naval force short of war to secure advantage or avert loss in an international dispute. For Cable, the essential issue was not necessarily the actual opening of fire. The central point was that the warship itself could become the visible, mobile and credible embodiment of political will.

According to Cable, sea power could be employed to create a fait accompli or to reverse an existing one; to compel an adversary to alter its behavior or policy; to generate additional options during an escalating crisis, thereby gaining time while maintaining forces in the area capable of intervening in subsequent developments; and, finally, to demonstrate political resolve and strategic intent through the visible presence of warships. The essence of all these functions can be reduced to a single strategic principle: to compel without going to war.

Maritime Coercion in the Age of Empires

The nineteenth century was replete with classic examples of gunboat diplomacy. The Don Pacifico Affair of 1850 began after Greece refused to compensate Don Pacifico, a Jewish businessman and British subject whose house in Athens had been ransacked by a fanatical Orthodox Greek mob. Britain responded by employing its naval power to blockade Piraeus. Prime Minister Lord Palmerston defended the principle that a British subject, wherever he might be in the world, should enjoy the protection of British power, invoking the famous formula Civis Romanus sum. Despite objections from France and Russia, Britain ultimately forced Greece to retreat from its position. The crisis became a classic example of gunboat diplomacy in which British naval superiority was transformed, without a declaration of war, into economic pressure and ultimately into a diplomatic outcome.

Another prominent example came in 1853–1854, when Commodore Matthew Perry arrived with his fleet of “Black Ships” and compelled isolationist Japan to open its doors to trade with the United States. Similarly, in 1902, Britain, Germany and Italy imposed a naval blockade on Venezuela after the country defaulted on its foreign debts. Venezuelan warships were seized and ports blockaded, eventually compelling Caracas to reach arrangements concerning repayment. This became another classic example of gunboat diplomacy: the use of sea power for debt enforcement and political coercion. In essence, the targeted state was confronted with a stark choice—either accept the political demands imposed upon it or face the consequences that superior naval power operating off its coast could inflict.

The fundamental condition that made this system effective was the military asymmetry between the great power and the targeted state. Most coastal states possessed neither a navy capable of challenging an adversary on the high seas nor long-range coastal-defense systems, precision-guided missiles, unmanned aerial vehicles, submarines or sophisticated mine-warfare capabilities. The warship was therefore not merely a military platform; it was the armed, visible and mobile embodiment of political will.

The Cold War and Aircraft Carrier Diplomacy

After the Second World War, gunboat diplomacy changed its form rather than disappearing. The advent of nuclear weapons made direct military confrontation between the great powers extraordinarily dangerous, yet crises, proxy wars and limited conflicts continued across the geopolitical periphery. It was precisely within this environment that naval forces became highly effective instruments for political pressure, signaling and demonstrations of power through conventional military capabilities. The deployment of a carrier strike group (CSG) to a crisis zone, the movement of an amphibious force towards a hostile coastline, or the deployment of nuclear-powered submarines into a region could convey a powerful political and strategic message.

The Taiwan Strait crises of the 1950s, the 1958 Lebanon crisis, the maritime quarantine imposed during the 1962 Cuban Missile Crisis, the deployment of the USS Enterprise task group to the Bay of Bengal during the 1971 Indo-Pakistani War, and the dispatch of two US aircraft carrier groups to the region during the 1995–1996 Taiwan Strait Crisis were different manifestations of the same tradition. Even throughout the proxy wars of the Cold War, gunboat diplomacy remained very much alive.

What had changed was principally the scale and technological character of the instrument. The gunboats and cruisers of the imperial age had increasingly given way to aircraft carriers, nuclear-powered submarines, amphibious forces and long-range naval aviation. Yet the underlying strategic logic remained unchanged: to translate the presence and potential firepower of naval forces into political influence without crossing the threshold into full-scale war.

Türkiye and Cyprus, 1974

Türkiye’s 1974 Cyprus Peace Operation also constitutes an important example of the ability of sea power to generate political outcomes. Here, however, the boundary established by Cable’s definition must be drawn carefully. Once the operation entered the actual phases of amphibious landing and combat, it moved beyond gunboat diplomacy in the classical sense and became a direct military operation. Prior to that threshold, however, the deployment of naval forces, the assembly of the amphibious force and the presence of the Turkish Navy around the island corresponded closely to Cable’s concept of limited naval force employed as an instrument of political pressure and as a demonstration of political resolve. Indeed, Cable would later discuss the 1974 Cyprus intervention alongside the Falklands case when examining the impact of limited naval force upon political decision-making. From Türkiye’s perspective, the fundamental lesson of Cyprus was clear: sea power could be employed not only to fight at sea, but also to alter the political outcome on land.

The Unipolar Era and the Golden Autumn of Gunboat Diplomacy

The unipolar era that emerged following the dissolution of the Soviet Union opened a new and exceptionally favorable strategic space for gunboat diplomacy. American aircraft carriers no longer faced a Soviet Navy capable of challenging them on a comparable global scale. From the Adriatic to the Persian Gulf, and from the Mediterranean to the Indian Ocean, US naval power enjoyed an extraordinary degree of freedom of maneuver. The approach of an American carrier strike group towards the coast of a targeted state was a major political event. This was because the aircraft carrier carried not only combat aircraft, but also the political will of the United States. Yet this era was not destined to last indefinitely.

The Arming of the Littorals and the End of the Era of Cost-Free Approach

One of the most significant military transformations of the twenty-first century has taken place not so much aboard warships as along the littorals themselves. Long-range anti-ship missiles, ballistic and cruise missiles, quiet diesel-electric submarines, naval mines, unmanned aerial vehicles, loitering munitions, unmanned surface vessels, satellite reconnaissance systems, and network-centric detection, tracking and targeting capabilities have provided coastal states—many of which were previously extremely vulnerable—with the means to threaten the highly expensive platforms of major naval powers.

Therefore, the traditional asymmetry in the cost of armed conflict and warfare has begun to reverse. A warship worth billions of dollars can now be threatened by a missile, UAV or unmanned system costing hundreds or even thousands of times less. The privilege of approaching an adversary’s coast with relative impunity—the psychological foundation upon which classical gunboat diplomacy largely rested—has therefore begun to erode.

The area-denial and anti-access capabilities employed by Iran in the Persian Gulf and Arabian Sea, and by the Houthis in the Red Sea, provide striking contemporary examples of this transformation. In and around the Strait of Hormuz, distributed force structures combining shore-based missiles, UAVs, naval mines and small attack craft have constrained the freedom of maneuver of major surface combatants. The Russia–Ukraine War in the Black Sea has produced similar results. Despite lacking a powerful conventional navy, Ukraine has been able to impose significant constraints on the freedom of maneuver of the Russian Black Sea Fleet through the employment of coastal missile systems, unmanned surface vessels and long-range UAVs. The strategic implication is profound. In the littoral battlespace of the twenty-first century, command of the sea and the ability to approach an adversary’s coast without prohibitive cost are no longer synonymous.

Sanctions Join the Gunboat

As the operational cost of classical gunboat diplomacy and maritime coercion has increased, another mechanism of coercion has progressively moved to the forefront. This system, which may be described as sanctions diplomacy, differs considerably from its predecessor. In the past, a warship would be dispatched off the port of the targeted state. Today, a tanker carrying that state’s oil may, without authorization from the UN Security Council, be labelled part of a “shadow fleet,” deprived of insurance coverage, excluded from financial systems, denied access to ports, prevented from obtaining services from classification societies, and placed—together with its owning or operating companies—on sanctions lists. In this manner, some of the functions once performed by a physical naval blockade are increasingly being exercised through instruments of financial and legal blockade.

The sanctions regimes imposed against Russia and Iran constitute the largest contemporary laboratories of this transformation. Hundreds of vessels have been placed on Western sanctions lists on the grounds that they transport Russian oil. In response, Russia and the actors continuing to trade with it have developed alternative insurance, flagging, corporate and payment mechanisms, creating a vast maritime transportation network commonly described as the “shadow fleet.” As a result, a sanctions confrontation that initially took place primarily within the financial and legal domains has gradually begun to return to the physical maritime domain.

From Sanctions to Physical Interdiction

The year 2026 marked an important threshold in the progressive transformation of unilateral economic sanctions into physical coercion at sea. During the wars involving Ukraine and Iran, merchant vessels placed on sanctions lists by the United States, the United Kingdom and certain European states faced not only restrictions involving ports, insurance and financing, but also the prospect of being stopped, inspected and boarded with the support of warships and military helicopters. Vessels alleged to have links with Russia and Iran—and described in Western terminology as belonging to “shadow fleets”—became principal targets of these practices.

The fundamental legal issue is that the inclusion of a vessel on a sanctions list maintained by the United States, the European Union or the United Kingdom does not create a legal authority to interfere with a foreign-flagged merchant vessel on the high seas. These measures are not universal sanctions adopted by the UN Security Council, but unilateral or regional measures deriving from the domestic or collective legal frameworks of the states concerned. Consequently, the mere presence of a vessel on such a sanctions list does not automatically confer upon third states the right to stop and board that vessel on the high seas.

Article 110 of the United Nations Convention on the Law of the Sea (UNCLOS) is therefore of particular importance. On the high seas, the fundamental principle is the exclusive jurisdiction of the flag state. The right of a foreign warship to visit a merchant vessel is confined to a limited number of circumstances, including piracy, the slave trade, unauthorized broadcasting, statelessness, or situations involving false claims of nationality. Being a “sanctioned vessel” or a vessel belonging to a so-called “shadow fleet” is not among the grounds for the right of visit enumerated in Article 110.

The fact that Western states, rather than relying solely upon their sanction’s designations, have sought to justify certain interdictions by invoking statelessness, invalid registration or questionable flag status is itself an implicit recognition of this legal reality. Interference on the high seas with a validly flagged foreign merchant vessel solely because it appears on a US, EU or UK sanctions list therefore raises serious questions under international law. Any contrary approach would effectively transform unilateral sanctions regimes into a form of global maritime law-enforcement authority.

The military consequences of this development have already begun to emerge. Russia has started providing naval escorts to certain sanctioned merchant vessels, using warships to offer protection against potential Western interdiction attempts. The issue of economic sanctions is thereby moving directly into the sphere of military interaction between states. When a warship seeking to enforce sanctions encounters the warship of another state protecting a merchant vessel, an economic sanctions dispute can rapidly escalate into a serious maritime crisis. The transformation of unilateral sanctions lacking UN Security Council authorization into physical interdiction on the high seas—through an expansive interpretation of the limited exceptions provided under UNCLOS—risks eroding both the freedom of the high seas and the principle of exclusive flag-state jurisdiction. Methods employed today against Russia or Iran may tomorrow be invoked by other states for entirely different purposes.

One of the major struggles of the coming era will therefore take place not merely between ships at sea, but over a much more fundamental question: under whose rules, and according to which body of law, will the high seas be governed?

Conclusion: The New Order of Maritime Coercion in the Twenty-First Century

Viewed across nearly two centuries of historical evolution, the instruments of maritime coercion have not disappeared. On the contrary, they have changed form and become increasingly complex. In the nineteenth century, the appearance of a warship off the coast of a targeted state could by itself be sufficient to influence political decision-making. In the twentieth century, carrier strike groups assumed the same function on a vastly greater scale. In the first quarter of the twenty-first century, however, sea power is becoming part of a new coercive mechanism integrating naval forces with sanctions lists, financial systems, insurance regimes, port restrictions, satellite surveillance and, when required, military forces capable of physical interdiction at sea.

Gunboat diplomacy itself has not disappeared either. China’s large-scale military exercises around Taiwan and the persistent naval presence of the United States and its allies in the Western Pacific, Eastern Mediterranean and Persian Gulf are contemporary manifestations of the same strategic tradition. Yet gunboat diplomacy is no longer exercised merely through the visible presence of warships. The declaration of extensive exercise areas through NAVTEX and similar navigational and safety notifications, the establishment of danger areas, the creation of maritime zones that vessels are temporarily advised to avoid or whose practical accessibility is restricted, and the use of intensive air and naval exercises to exert pressure upon the commercial and military use of particular maritime spaces have all become part of the modern repertoire of maritime coercion. Although such measures do not legally constitute a blockade or a declaration of sovereignty, depending upon their duration, geographical extent and the degree to which they are supported by military force, they can become instruments for creating faits accomplis at sea and conveying political messages.

At the same time, the overwhelming superiority upon which the old model depended has been eroded. Littoral regions are no longer as defenseless as they were during the nineteenth and twentieth centuries. Long-range anti-ship missiles, ballistic and hypersonic systems, submarines, unmanned platforms and shore-based sensor networks mean that while the approach of an aircraft carrier or major surface combatant towards an adversary’s coast continues to convey a powerful political message, it simultaneously generates an increasingly serious operational—and potentially strategic—risk. Particularly for Western naval powers, the center of gravity of coercion is therefore shifting from sinking the ship to preventing it from trading; from destroying the port to denying access to it; and from classical naval blockade to financial, legal and technological blockade.

Yet the boundary between economic and legal coercion on the one hand and the physical employment of naval power on the other is also becoming increasingly blurred. As demonstrated during the wars involving Ukraine and Iran, the stopping or boarding of merchant vessels placed on unilateral sanctions lists, supported by warships and military helicopters, represents the most dangerous stage of this transformation. The conversion of sanctions lacking UN Security Council authorization into physical interdiction on the high seas directly engages the principles of freedom of the high seas and the exclusive jurisdiction of the flag state. Of particular importance is the fact that Article 110 of UNCLOS contains no independent category granting a right of visit merely because a vessel is “sanctioned” or belongs to a so-called “shadow fleet.” Russia’s decision to provide naval escorts to certain merchant vessels further demonstrates how dramatically the distance between economic sanctions and direct military encounters between states has narrowed.

Today, the gunboat is still at sea, but it no longer sails alone. Alongside it sail the bank, the insurance company, the satellite, the sanctions list, the port state and the lawyer, while NAVTEX notifications delineate exercise and danger areas across the maritime domain. The gunboat diplomacy of the nineteenth century is thus evolving into a multilayered system of maritime coercion in the twenty-first century, in which economic, legal, technological and military instruments are employed in combination.

Yesterday, the warship was the most effective ambassador in grey. Today, when the warship puts to sea, economic and legal power sail with it.

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This article was originally published on Mavi Vatan.

Ret Admiral Cem Gürdeniz, Writer, Geopolitical Expert, Theorist and creator of the Turkish Bluehomeland (Mavi Vatan) doctrine. He served as the Chief of Strategy Department and then the head of Plans and Policy Division in Turkish Naval Forces Headquarters. As his combat duties, he has served as the commander of Amphibious Ships Group and Mine Fleet between 2007 and 2009. He retired in 2012. He established Hamit Naci Blue Homeland Foundation in 2021. He has published numerous books on geopolitics, maritime strategy, maritime history and maritime culture. He is also a honorary member of ATASAM. 

He is a Research Associate of the Centre for Research on Globalization (CRG).

Featured image is from the author

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