Territorial Waters Under International Law: The 12-Mile Rule
Introduction
Territorial waters under international law are governed by the legal regime of the territorial sea. Under the 1982 United Nations Convention on the Law of the Sea (UNCLOS), a coastal State’s sovereignty extends beyond its land territory and internal waters to an adjacent belt of sea, including the airspace above it and its bed and subsoil. A State may establish the breadth of that territorial sea up to a maximum of 12 nautical miles, measured from baselines determined in accordance with the Convention (United Nations, 1982, arts 2–3).
UNCLOS uses the term “territorial sea,” although “territorial waters” remains common in public and governmental usage. The distinction is useful because the latter expression has sometimes been used more loosely for waters subject to different forms of coastal-State authority. The 12-nautical-mile rule is also frequently simplified. The territorial sea is not measured mechanically from the visible shoreline: the normal baseline is generally the low-water line along the coast, while UNCLOS permits other baseline methods in defined geographical circumstances (United Nations, 1982, arts 5–14).
Sovereignty in the territorial sea is extensive, but it is not unlimited. UNCLOS expressly provides that it is exercised subject to the Convention and other rules of international law. One of the principal qualifications is the right of innocent passage. Ships of all States may pass through the territorial sea when their passage complies with the Convention; it must be continuous and expeditious, subject to limited exceptions, and must not be prejudicial to the peace, good order, or security of the coastal State (United Nations, 1982, arts 17–19).
The territorial sea must also be distinguished from neighboring maritime zones. Coastal-State sovereignty within the territorial sea differs from the limited control permitted in the contiguous zone and from the sovereign rights and jurisdiction recognized in the exclusive economic zone (United Nations, 1982, arts 33, 55–57). The legal position consequently depends not only on the 12-nautical-mile limit, but also on how the applicable baseline is drawn, what authority the coastal State may exercise, and how international law protects navigation through the waters concerned.
1. What Territorial Waters Mean Under International Law
“Territorial waters” is a common expression, but the governing term in the United Nations Convention on the Law of the Sea (UNCLOS) is “territorial sea.” Article 2 provides that the sovereignty of a coastal State extends beyond its land territory and internal waters to an adjacent belt of sea described as the territorial sea. That sovereignty also covers the airspace above the territorial sea and its bed and subsoil (United Nations, 1982, art. 2).
The distinction in terminology helps prevent confusion between maritime zones that carry different legal consequences. Internal waters and the territorial sea are both subject to coastal-State sovereignty, but their legal regimes are not identical. In particular, foreign ships generally enjoy a right of innocent passage through the territorial sea, whereas no equivalent general right applies throughout internal waters.
Farther seaward, the legal character changes. The contiguous zone does not place the sea itself under coastal-State sovereignty; instead, UNCLOS permits specified forms of control relating to customs, fiscal, immigration, and sanitary laws. In the exclusive economic zone, the coastal State possesses sovereign rights over specified resources and jurisdiction over defined activities, while other States retain important freedoms recognized by the Convention (United Nations, 1982, arts 33, 56).
The territorial sea is thus a distinct legal zone, not a general label for every area in which a coastal State exercises authority. Its defining feature is sovereignty subject to international law, including rules protecting navigation. The geographical extent of that sovereignty depends on the breadth claimed by the coastal State and, critically, on the baselines from which the territorial sea is measured.
2. The 12-Nautical-Mile Limit
Article 3 of UNCLOS gives every State the right to establish the breadth of its territorial sea up to a limit not exceeding 12 nautical miles, measured from baselines determined in accordance with the Convention (United Nations, 1982, art. 3). The language establishes a maximum permissible breadth. It does not itself assign an automatic 12-nautical-mile territorial sea to every coastal State irrespective of national legislation, geography, or overlapping claims.
The internationally standardized nautical mile is 1,852 meters, so 12 nautical miles equal approximately 22.2 kilometers or 13.8 statute miles. The nautical mile became the standard unit for maritime navigation because of its relationship to geographical coordinates and angular measurement on the Earth’s surface. Its use also gives maritime boundaries a common unit that is independent of national systems of land measurement.
The 12-nautical-mile rule was not always the accepted maximum. During the nineteenth and early twentieth centuries, three-nautical-mile territorial seas were widely used, alongside broader claims by other States. The three-mile rule was historically associated with earlier ideas linking effective coastal control to the range of shore-based cannon, but the relationship between the cannon-shot rule and later fixed-distance claims was neither uniform nor mechanically direct.
Attempts to codify a universal breadth repeatedly failed before UNCLOS. The 1930 Hague Codification Conference could not secure agreement on the extent of the territorial sea. The 1958 Convention on the Territorial Sea and the Contiguous Zone codified important rules concerning the regime but left its maximum breadth unresolved, and the Second United Nations Conference on the Law of the Sea in 1960 again failed to produce the necessary agreement.
By the Third United Nations Conference on the Law of the Sea, State practice had moved toward wider territorial seas, while major maritime States continued to press for legal protection of navigation. The eventual settlement combined a maximum territorial sea of 12 nautical miles with separate regimes for the contiguous zone, exclusive economic zone, and navigation through international straits. Article 3 of the 1982 Convention expresses that compromise in its modern form.
3. Where the 12 Miles Begin: Maritime Baselines
The familiar statement that territorial waters extend 12 nautical miles “from the coast” is only an approximation. Legally, the territorial sea is measured from baselines established under UNCLOS. Article 4 defines its outer limit by reference to points located at a distance from the nearest point of the baseline equal to the breadth of the territorial sea claimed by the coastal State (United Nations, 1982, art. 4).
Baselines do not always coincide with the coastline as it appears on an ordinary map. UNCLOS uses the low-water line as the normal method but provides other rules for particular geographical conditions and coastal features. The location of the baseline can consequently affect the amount of maritime space lying on both its landward and seaward sides.
Its significance extends beyond the territorial sea. The contiguous zone and the exclusive economic zone are also measured from the baselines used to calculate the territorial sea. A baseline is thus a legal reference line from which several major maritime entitlements are projected.
3.1 The Normal Baseline and the Low-Water Line
Article 5 of UNCLOS establishes the normal rule. Unless the Convention provides otherwise, the normal baseline is the low-water line along the coast as marked on large-scale charts officially recognized by the coastal State (United Nations, 1982, art. 5).
The low-water line is not the same thing as the water’s edge observed at any particular moment. Along tidal coasts, the visible shoreline moves as water levels change. A rule based on the momentary edge of the sea would make maritime measurement unstable, while the charted low-water line provides a legally identifiable reference.
Where the normal baseline applies, the territorial sea is projected seaward from that line. If the coastal State claims the full breadth permitted by UNCLOS, the outer limit lies 12 nautical miles from the nearest points of the applicable baseline. Because coastlines are rarely geometrically regular, the resulting outer limit may curve and shift with coastal geography rather than appearing as a perfectly parallel offshore line.
3.2 When Special Baseline Rules Apply
UNCLOS permits different baseline methods where coastal geography makes exclusive reliance on the normal low-water line inappropriate. Article 7 allows straight baselines joining appropriate points where the coastline is deeply indented and cut into or where a fringe of islands lies along the coast in its immediate vicinity. It also contains a rule for highly unstable coastlines associated with deltas and other natural conditions (United Nations, 1982, art. 7).
Straight baselines remain subject to legal limits. They must not depart to any appreciable extent from the general direction of the coast, and the sea areas lying within them must be sufficiently closely linked to the land domain to be subject to the regime of internal waters. UNCLOS also restricts the use of low-tide elevations as baseline points and prohibits straight-baseline systems that cut off another State’s territorial sea from the high seas or an exclusive economic zone.
The leading judicial authority is Fisheries (United Kingdom v. Norway). The dispute concerned Norway’s method of drawing baselines along a highly indented coast bordered by a fringe of islands. The International Court of Justice upheld the Norwegian system against the United Kingdom’s challenge, emphasizing the special geographical character of the coastline and the need for the method used to remain connected to the general direction of the coast and the relationship between land and adjoining waters (ICJ, 1951). The judgment remains central to the legal understanding of straight baselines.
Separate UNCLOS provisions address other coastal features. Where a river flows directly into the sea, Article 9 permits a straight baseline across its mouth between points on the low-water line of the banks. Article 10 establishes rules for juridical bays whose coasts belong to a single State, including a maximum closing line of 24 nautical miles where the conditions specified in the Convention are satisfied. Historic bays are not governed by those rules (United Nations, 1982, arts 9–10).
Harbors and low-tide elevations receive distinct treatment. Article 11 allows the outermost permanent harbor works forming an integral part of the harbor system to be regarded as part of the coast for delimiting the territorial sea, while excluding offshore installations and artificial islands from that category. Under Article 13, the low-water line of a low-tide elevation may be used as a baseline where the feature lies wholly or partly within the territorial-sea breadth measured from the mainland or an island; if it lies wholly beyond that distance, it has no territorial sea of its own (United Nations, 1982, arts 11, 13).
UNCLOS also permits coastal States to combine the baseline methods provided in the Convention where different geographical conditions exist along different parts of the coast. Baseline law is consequently not a technical afterthought. It determines where measurement begins and can shape the spatial reach of several forms of maritime jurisdiction.
4. What Coastal-State Sovereignty Covers
Article 2 of UNCLOS places the territorial sea under coastal-State sovereignty. That sovereignty extends to the water column, the bed and subsoil beneath it, and the airspace above. It is exercised subject to the Convention and other rules of international law (United Nations, 1982, art. 2).
Within the territorial sea, the coastal State has broad authority to regulate activities connected with its territory and maritime interests. It may control the exploitation of living and non-living resources and regulate activities such as fishing, navigation, environmental protection, and marine scientific research in accordance with international law. Foreign vessels have no general right to exploit resources merely because they possess a right of passage.
UNCLOS gives more specific content to that authority in the context of foreign shipping. Article 21 permits the coastal State to adopt laws relating to the safety of navigation and maritime traffic, protection of navigational aids and cables, conservation of living resources, prevention of infringement of fisheries laws, environmental protection, marine scientific research, and customs, fiscal, immigration, and sanitary matters. Ships exercising innocent passage are required to comply with applicable coastal-State laws adopted consistently with the Convention (United Nations, 1982, art. 21).
Those regulatory powers are limited by the navigational rights recognized in UNCLOS. Article 24 prohibits the coastal State from hampering innocent passage except in accordance with the Convention and from imposing requirements that have the practical effect of denying or impairing that right. The State must also give appropriate publicity to dangers to navigation within its territorial sea of which it has knowledge (United Nations, 1982, art. 24).
Security interests receive separate protection. Article 25 allows the coastal State to take necessary steps to prevent passage that is not innocent. It may also temporarily suspend innocent passage in specified areas when suspension is essential for the protection of its security, including for weapons exercises, but only without discrimination among foreign ships and after the suspension has been duly published (United Nations, 1982, art. 25).
Sovereignty also extends vertically. Under UNCLOS Article 2 and the 1944 Convention on International Civil Aviation, the airspace above the territorial sea forms part of the coastal State’s sovereign airspace. The right of innocent passage created by UNCLOS applies to ships and does not establish a corresponding general right of foreign aircraft to fly through that airspace. International overflight rights must arise from another applicable legal basis, such as treaty arrangements or the coastal State’s authorization (Chicago Convention, 1944, arts 1–2).
Foreign State vessels introduce further qualifications. UNCLOS preserves the immunities of warships and other government ships operated for non-commercial purposes, while allowing a coastal State to require a warship that disregards applicable passage laws and regulations to leave the territorial sea immediately (United Nations, 1982, arts 30, 32). These rules confirm that sovereignty over the territorial sea is extensive but operates within a legal regime that also protects navigation and certain immunities of foreign vessels.
5. Innocent Passage Through the Territorial Sea
Coastal-State sovereignty over the territorial sea is qualified by the right of innocent passage. Article 17 of UNCLOS provides that, subject to the Convention, ships of all States, whether coastal or landlocked, enjoy that right through the territorial sea (United Nations, 1982, art. 17). For vessels falling within the treaty regime, passage is a legal entitlement rather than a privilege granted at the coastal State’s discretion, although separate questions arise over conditions imposed by some States on foreign warships.
The regime accommodates territorial sovereignty and international navigation within the same maritime space. A foreign ship remains in waters under the sovereignty of the coastal State, but the coastal State must respect passage that satisfies UNCLOS. Conversely, the vessel does not acquire the broader freedoms associated with the high seas merely because it is navigating rather than exploiting resources.
Innocent passage is also distinct from transit passage through certain international straits. It applies to ships rather than aircraft, is subject to coastal regulation within the limits set by UNCLOS, and may be temporarily suspended in defined circumstances. Transit passage, by contrast, includes overflight and cannot be suspended under the Convention.
5.1 What Counts as Passage
Article 18 defines passage by reference to the purpose and manner of navigation. It includes traversing the territorial sea without entering internal waters or calling at a roadstead or port facility outside internal waters. It also covers navigation undertaken to enter or leave internal waters or to call at, or depart from, such a roadstead or port facility (United Nations, 1982, art. 18).
These forms are commonly described as lateral passage and inward or outward passage. The distinction later becomes relevant to coastal-State jurisdiction because UNCLOS treats a vessel merely crossing the territorial sea differently in some respects from one that has left the coastal State’s internal waters.
Passage must be continuous and expeditious. Stopping and anchoring are compatible with passage only when incidental to ordinary navigation, required by force majeure or distress, or undertaken to assist persons, ships, or aircraft in danger or distress. Prolonged activity unrelated to navigation cannot acquire the protection of innocent passage simply because a vessel characterizes its presence as transit.
5.2 When Passage Ceases to Be Innocent
Under Article 19, passage is innocent only so long as it is not prejudicial to the peace, good order, or security of the coastal State. The Convention gives that standard more concrete content by identifying activities that make passage prejudicial when carried out in the territorial sea (United Nations, 1982, art. 19).
Among them are threats or uses of force against the sovereignty, territorial integrity, or political independence of the coastal State, or force otherwise contrary to the principles of international law embodied in the UN Charter. Weapons exercises, intelligence collection prejudicial to the coastal State’s defense or security, propaganda aimed at affecting its defense or security, and the launching, landing, or taking on board of aircraft or military devices are likewise inconsistent with innocent passage.
Article 19 also addresses non-military conduct. Fishing, research or survey activities, serious acts of willful pollution contrary to UNCLOS, interference with communications or other facilities, and loading or unloading commodities, currency, or persons contrary to specified coastal laws may render passage non-innocent. The final category covers any other activity not having a direct bearing on passage.
UNCLOS focuses on conduct rather than nationality or vessel classification alone. Warship status is not itself listed in Article 19 as a ground of non-innocence, nor does merchant status guarantee that passage remains innocent. The legal character of the passage depends on the vessel’s conduct and the rules applicable to its category, with additional questions of immunity arising for warships and other government vessels.
5.3 Coastal Regulation and Temporary Suspension
The right of innocent passage does not eliminate coastal regulatory authority. Article 21 permits laws concerning navigational safety and maritime traffic, protection of navigational aids, cables and pipelines, conservation of living resources, fisheries, environmental protection, marine scientific research and hydrographic surveys, and the prevention of infringements of customs, fiscal, immigration, and sanitary laws (United Nations, 1982, art. 21).
That authority is not unlimited. Coastal legislation governing passage may not apply to the design, construction, manning, or equipment of foreign ships unless it gives effect to generally accepted international rules or standards. Foreign vessels exercising innocent passage must comply with valid coastal laws and with generally accepted international regulations concerning the prevention of collisions at sea.
Article 22 permits the coastal State, where necessary for navigational safety, to require foreign ships to use designated sea lanes and traffic separation schemes. Tankers, nuclear-powered ships, and vessels carrying nuclear or other inherently dangerous or noxious substances may be required to confine their passage to prescribed sea lanes. Article 23 separately requires nuclear-powered vessels and ships carrying specified dangerous substances to carry the documents and observe the special precautionary measures established for them by international agreements (United Nations, 1982, arts 22–23).
Regulation may not be used to neutralize the right itself. Article 24 prohibits requirements that have the practical effect of denying or impairing innocent passage and prohibits discrimination in form or fact against the ships of particular States or against vessels carrying cargoes to, from, or on behalf of particular States. Article 26 similarly prevents charges from being imposed merely for passage, although non-discriminatory charges may be levied for specific services rendered.
The coastal State may take necessary steps to prevent passage that is not innocent. It may also suspend innocent passage temporarily in specified areas where suspension is essential for the protection of its security, including for weapons exercises. Such suspension must not discriminate among foreign ships and takes effect only after due publication (United Nations, 1982, art. 25).
6. Jurisdiction Over Foreign Ships
Territorial sovereignty does not give the coastal State unrestricted enforcement power over every event aboard a foreign ship. UNCLOS separates sovereignty over the territorial sea from the circumstances in which coastal authorities may arrest persons, investigate crimes, or enforce civil claims on board vessels engaged in passage.
For crimes committed aboard a foreign ship during passage, Article 27 states that coastal criminal jurisdiction should not ordinarily be exercised to arrest a person or conduct an investigation. Intervention is permitted where the consequences of the crime extend to the coastal State, the crime disturbs the peace of the country or the good order of the territorial sea, assistance has been requested by the master or by a diplomatic or consular representative of the flag State, or action is necessary to suppress illicit traffic in narcotic drugs or psychotropic substances (United Nations, 1982, art. 27).
A different rule applies when a foreign ship passes through the territorial sea after leaving internal waters. Article 27(2) preserves the coastal State’s right to take steps authorized by its law for arrest or investigation on board such a vessel. The provision is not confined to offenses committed while the ship was inside internal waters.
By contrast, where a ship proceeding from a foreign port is merely passing through the territorial sea without entering internal waters, Article 27(5) generally prevents coastal authorities from acting on board in relation to a crime committed before the vessel entered the territorial sea. That restriction does not displace cases arising under Part XII of UNCLOS or violations of laws and regulations adopted in accordance with Part V. The Convention also requires due regard to navigational interests when coastal authorities decide whether, or how, to make an arrest.
Civil jurisdiction is narrower in another way. Article 28 provides that the coastal State should not stop or divert a foreign ship passing through the territorial sea merely to exercise civil jurisdiction over a person on board. Execution against or arrest of the vessel for civil proceedings is generally permitted only for obligations or liabilities assumed or incurred by the ship itself in the course or for the purpose of its voyage through the coastal State’s waters. That restriction does not prejudice the coastal State’s right, under its law, to levy execution against or arrest a foreign ship lying in the territorial sea or passing through it after leaving internal waters (United Nations, 1982, art. 28).
The resulting regime is more precise than the proposition that territorial sovereignty entails complete enforcement jurisdiction. A coastal State possesses sovereignty over its territorial sea, but UNCLOS regulates when that sovereignty may be exercised directly against foreign ships and persons aboard them. The vessel’s route, the timing and effects of an offense, and whether it entered or left internal waters can alter the legal position.
7. Warships, Submarines, and State Vessels
UNCLOS gives warships a distinct legal status. Article 29 defines a warship by reference to its membership in a State’s armed forces, external marks identifying its nationality, command by a duly commissioned officer, and a crew subject to regular armed-forces discipline (United Nations, 1982, art. 29). These characteristics distinguish warships from private and commercial vessels for purposes including immunity and enforcement.
Article 32 preserves the immunities of warships and other government ships operated for non-commercial purposes, subject to the provisions identified in Part II. Coastal authorities cannot exercise enforcement jurisdiction over such vessels in the same manner as over ordinary merchant shipping. Immunity, however, does not exempt them from every rule governing navigation through the territorial sea.
If a warship fails to comply with coastal-State laws and regulations concerning passage and disregards a request for compliance, Article 30 permits the coastal State to require it to leave the territorial sea immediately. Article 31 places international responsibility on the flag State for loss or damage resulting from non-compliance by a warship or other government ship operated for non-commercial purposes with applicable coastal laws, UNCLOS, or other rules of international law (United Nations, 1982, arts 30–31).
Submarines and other underwater vehicles are subject to an express condition when exercising innocent passage. Article 20 requires them to navigate on the surface and show their flag. That rule is specific to innocent passage and should not be transferred automatically to the different regime of transit passage through international straits, under which vessels may proceed in their normal modes of continuous and expeditious transit.
Prior authorization for foreign warships remains more difficult. Article 17 appears within the Convention’s “Rules Applicable to All Ships,” while Articles 29–32 expressly regulate warships without creating a general requirement of prior coastal authorization. Many maritime States interpret that structure as protecting innocent passage for warships without advance consent. A number of coastal States, however, maintain requirements of prior notification or authorization, producing continuing disagreement in State practice over the legality of such conditions.
The treaty question is distinct from the precise scope of customary international law. In Corfu Channel, the International Court of Justice held that States in time of peace had a customary right to send warships through straits used for international navigation between two parts of the high seas without prior coastal-State authorization, provided that the passage was innocent (ICJ, 1949). The Court did not decide whether an equivalent customary right applies to warships throughout every ordinary territorial sea. Corfu Channel is consequently strong authority for passage through qualifying international straits, but it does not by itself settle the broader controversy.
8. Territorial Sea, EEZ, and Adjacent Maritime Zones
Internal waters lie on the landward side of the territorial-sea baseline and are subject to coastal-State sovereignty. They differ from the territorial sea because no equivalent general right of innocent passage applies throughout internal waters. Article 8(2) creates a limited exception where straight baselines enclose waters that had not previously been considered internal waters, in which case a right of innocent passage continues to exist (United Nations, 1982, art. 8).
The contiguous zone may extend no farther than 24 nautical miles from the baselines used to measure the territorial sea. It is not subject to coastal-State sovereignty. Article 33 instead permits the coastal State to exercise the control necessary to prevent infringement of its customs, fiscal, immigration, or sanitary laws within its territory or territorial sea and to punish infringements committed there (United Nations, 1982, art. 33).
An exclusive economic zone may extend up to 200 nautical miles from the relevant baselines and has a different legal character again. The coastal State enjoys sovereign rights over natural resources and certain economic activities, together with jurisdiction over specified matters such as artificial islands and installations, marine scientific research, and protection of the marine environment. Other States retain freedoms of navigation and overflight and the laying of submarine cables and pipelines, subject to UNCLOS (United Nations, 1982, arts 56–58).
The high-seas regime applies outside the EEZ, territorial sea, internal waters, and archipelagic waters, while UNCLOS separately preserves specified high-seas freedoms within the EEZ. “International waters” is not a defined maritime zone under UNCLOS and is used loosely in public discourse. It may refer to areas beyond the territorial sea or, more narrowly, to the high seas, depending on context. For legal analysis, the specific UNCLOS zone should be identified instead.
9. When Territorial Seas of Two States Overlap
A State entitled to a territorial sea cannot necessarily project the full claimed breadth against a neighboring State. Where opposite or adjacent coasts are close enough for territorial-sea claims to overlap, the competing entitlements require delimitation. States remain free to establish the boundary by agreement.
In the absence of agreement, Article 15 provides that neither State is entitled, as against the other, to extend its territorial sea beyond the median line. Every point on that line is equidistant from the nearest points on the baselines from which the breadth of each State’s territorial sea is measured. The rule does not apply where historic title or other special circumstances make a different delimitation necessary (United Nations, 1982, art. 15).
International jurisprudence confirms both the importance and the flexibility of this approach. In Maritime Delimitation and Territorial Questions between Qatar and Bahrain, the International Court of Justice treated the equidistance/special-circumstances rule applicable to territorial-sea delimitation as reflecting customary international law (ICJ, 2001). In Territorial and Maritime Dispute between Nicaragua and Honduras in the Caribbean Sea, the Court found that unstable coastal geography made ordinary equidistance impracticable for part of the boundary and employed a bisector method, while treating island territorial seas separately (ICJ, 2007).
Territorial-sea delimitation should not be assumed to follow exactly the same treaty methodology as delimitation of the EEZ and continental shelf. Article 15 contains an express median-line rule accompanied by specific qualifications. Articles 74 and 83, by contrast, require EEZ and continental-shelf boundaries to be established by agreement in accordance with international law in order to achieve an equitable solution. Judicial methods have influenced one another, but the treaty provisions begin from different formulations.
10. Treaty Rules and Customary International Law
UNCLOS binds its parties as treaty law, but many rules governing the territorial sea also have an existence in customary international law. This distinction becomes significant when a dispute involves a State not bound by a particular UNCLOS provision or when a treaty rule is invoked as evidence of an independently binding customary norm.
The sovereignty of the coastal State over the territorial sea predates the 1982 Convention. In Corfu Channel, the International Court of Justice treated Albanian territorial waters as subject to Albania’s sovereignty while simultaneously recognizing customary navigational rights through an international strait (ICJ, 1949). The territorial sea was thus already understood as sovereign maritime space subject to legal qualifications before UNCLOS codified the modern regime.
The 12-nautical-mile maximum is not solely a treaty rule. In Territorial and Maritime Dispute (Nicaragua v. Colombia), the International Court of Justice stated that UNCLOS Article 3 reflects customary international law concerning a coastal State’s right to establish a territorial sea extending up to 12 nautical miles (ICJ, 2012). That customary entitlement does not mean that every State automatically possesses a territorial sea of exactly that breadth; the breadth actually established remains a separate question.
The right of innocent passage also predates UNCLOS and was codified in the 1958 Convention on the Territorial Sea and the Contiguous Zone before being reformulated in the 1982 Convention (United Nations, 1958; United Nations, 1982). It does not follow, however, that every detail of Articles 17–32 has acquired an identical customary status. Particular treaty rules require separate examination of State practice and opinio juris before they can be treated as binding on States independently of UNCLOS.
Warship passage illustrates the need to keep treaty interpretation and custom distinct. Whether UNCLOS permits coastal States to require advance authorization or notification is a question about the meaning and structure of the Convention. Whether customary international law independently guarantees foreign warships innocent passage through ordinary territorial seas is a different inquiry, and State practice remains divided. Corfu Channel resolves the narrower customary position for qualifying international straits, not every aspect of naval passage through territorial waters.
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Conclusion
Territorial waters under international law are most precisely understood through the legal concept of the territorial sea. A coastal State may establish that zone up to 12 nautical miles from baselines determined according to international law. The distance is not measured simply from the visible shoreline, and coastal geography can affect both the location of the baseline and the resulting outer limit.
Within the territorial sea, the coastal State exercises sovereignty over the water column, seabed, subsoil, and airspace. That sovereignty carries extensive regulatory and enforcement powers, but it is exercised within a legal regime that protects innocent passage, limits certain forms of jurisdiction over passing foreign ships, and preserves the immunities of warships and other qualifying government vessels.
The territorial sea is consequently defined by more than the familiar 12-nautical-mile figure. It differs from internal waters, the contiguous zone, the EEZ, and the high seas because each allocates authority differently between the coastal State and other States. The central legal proposition is one of qualified sovereignty: the coastal State possesses territorial authority, but that authority operates alongside international rules protecting navigation and other lawful uses of the sea.
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