In 2025, a US Navy destroyer sailed through the South China Sea — in waters China claims as its own. China dispatched warships to “warn” it away. The US said it was exercising a legal right under international law. Both sides cited different interpretations of the same treaty: UNCLOS — the United Nations Convention on the Law of the Sea. Understanding it is no longer just for lawyers and diplomats. It’s essential knowledge for every NDA, CDS, AFCAT, and CAPF aspirant.
Key Takeaways
- UNCLOS has 168 signatories — called the “constitution of the oceans,” it governs all maritime zones globally (Wikipedia)
- Five zones: Internal Waters → Territorial Sea (12 nm) → Contiguous Zone (24 nm) → EEZ (200 nm) → High Seas
- Innocent passage (territorial sea) vs transit passage (international straits) — a critical exam distinction
- India requires prior notification for foreign warships — not in UNCLOS but asserted as national security right
- 2016 Hague Tribunal: China’s Nine-Dash Line claims lack UNCLOS foundation — China rejected the ruling
What Is UNCLOS and Why Does It Matter?
UNCLOS, adopted in 1982 and entering into force in 1994, is the foundational international treaty governing the world’s oceans — covering navigation rights, fishing, seabed mining, and military movement (Wikipedia). With 168 signatories, it is one of the most widely ratified international treaties in history. Every major maritime dispute today — South China Sea, Arctic, Strait of Hormuz — is argued using its language.
UNCLOS establishes five maritime zones with distinct sovereignty rights and navigation freedoms, creating a layered legal framework that balances state control with international freedom of movement — and is the legal basis for every Freedom of Navigation Operation (FONOP) conducted by major naval powers today (Marine Public).
What Are the Five Maritime Zones Under UNCLOS?
UNCLOS divides the ocean into five concentric zones measured from a country’s baseline — usually the low-water line along its coast (Marine Public). Each zone grants different rights to coastal states and different freedoms to other nations.
1. Internal Waters — Full Sovereignty
Waters on the landward side of the baseline — ports, harbours, rivers, bays. The coastal state has complete sovereignty here. Foreign ships have no automatic right of passage. Entry requires permission.
2. Territorial Sea — 12 Nautical Miles
The coastal state has full sovereignty — including over airspace, seabed, and subsoil. Other states enjoy the right of innocent passage: transit must be continuous, expeditious, and non-threatening. Submarines must surface and show flag.
3. Contiguous Zone — 24 Nautical Miles
A buffer zone where the coastal state can enforce customs, fiscal, immigration, and sanitary laws. Not full sovereignty — but meaningful control over who approaches its shores.
4. Exclusive Economic Zone (EEZ) — 200 Nautical Miles
The coastal state has sovereign rights over all natural resources — fish, oil, gas, minerals — and jurisdiction over marine research and environment (Britannica). But the EEZ is not territorial water — other nations retain full freedom of navigation and overflight. You can sail through freely; you can’t fish without permission.
5. High Seas — Beyond 200 nm
No state can claim sovereignty over any part of the high seas. All nations enjoy freedom of navigation, overflight, fishing, and scientific research. In 2023, the UN agreed on a High Seas Treaty adding marine protected areas and environmental protections to this zone (Wikipedia).
What Is the Difference Between Innocent Passage and Transit Passage?
This is one of the most frequently tested distinctions in defence and civil services exams — and the source of real-world maritime friction. Innocent passage applies in the territorial sea (0–12 nm); transit passage applies in international straits used for navigation between one part of the high seas or EEZ and another (Wikipedia).
| Feature | Innocent Passage | Transit Passage |
|---|---|---|
| Where it applies | Territorial Sea (0–12 nm) | International Straits |
| Submarines | Must surface and show flag | Can remain submerged |
| Aircraft overflight | Not covered | Covered — freedom of overflight |
| Can coastal state suspend? | Yes — temporarily | No — non-suspendable |
| Key example | Foreign warship near Indian coast | Any vessel through Strait of Hormuz / Malacca |
Under UNCLOS, instead of coastal states being able to block navigation through international straits, all ships — including warships and submarines — enjoy a non-suspendable right of transit passage, keeping critical chokepoints like the Strait of Hormuz and Strait of Malacca legally open to all vessels regardless of political tensions (UNCLOS Part III).
What Is India’s Position on Innocent Passage?
India ratified UNCLOS in 1995 but its domestic maritime law adds a condition not in the treaty: foreign warships and submarines must provide prior notification before entering India’s territorial waters under innocent passage (Dalvoy, 2025). India asserts this as a national security necessity. The US contests it, conducting FONOPs through Indian waters to challenge this position.
India’s prior notification requirement for foreign warships in its territorial sea — while not mandated by UNCLOS — reflects a broader pattern among developing maritime nations interpreting UNCLOS’s innocent passage provisions to include national security screening rights, a position the US actively challenges through Freedom of Navigation Operations (Dalvoy, 2025).
How Does the South China Sea Dispute Test UNCLOS?
China claims roughly 90% of the South China Sea within its “Nine-Dash Line” — overlapping with the EEZs of Vietnam, Philippines, Malaysia, Brunei, and Taiwan. In 2016, the Hague Tribunal ruled that China’s historic rights claims lack UNCLOS foundation and that China had violated the Philippines’ sovereign EEZ rights (DRAS). China rejected the ruling as “null and void” — exposing UNCLOS’s fundamental enforcement gap when a major power refuses to comply.
The 2016 Hague Tribunal established that features in the Spratly Islands aren’t entitled to EEZs under UNCLOS, and that China’s actions violated Philippines’ 200 nm sovereign rights — but with no enforcement mechanism, China simply ignored the ruling, making this the clearest test case of UNCLOS’s real-world limits (DRAS).
Exam Quick Reference: Key UNCLOS Facts
- Full form: United Nations Convention on the Law of the Sea
- Adopted: 1982 | In force: 1994 | Signatories: 168 | Known as: “Constitution of the Oceans”
- Territorial Sea: 12 nm — full sovereignty + innocent passage
- Contiguous Zone: 24 nm — customs/immigration enforcement
- EEZ: 200 nm — sovereign resource rights; free navigation by others
- High Seas: Beyond 200 nm — no sovereignty, open to all
- Innocent passage: Continuous, non-threatening; submarines must surface; can be suspended
- Transit passage: International straits; non-suspendable; submarines can submerge; aircraft covered
- India: Prior notification required for warships — not in UNCLOS, contested by US via FONOPs
- 2016 Hague ruling: China’s Nine-Dash Line invalid under UNCLOS — China rejected it
- 2023 High Seas Treaty: Marine protected areas + environmental assessments for international waters
- USA: Signed but never ratified UNCLOS
Frequently Asked Questions
What is UNCLOS and what does it govern?
UNCLOS (United Nations Convention on the Law of the Sea), adopted in 1982 and in force since 1994, governs all ocean activity — navigation, resources, marine environment, and disputes. Its 168 signatories make it one of the most widely ratified treaties in history, defining five maritime zones from internal waters to the high seas (Wikipedia).
What is the difference between innocent passage and transit passage?
Innocent passage applies in the territorial sea (0–12 nm): ships must transit continuously and non-threateningly; submarines must surface; coastal states can temporarily suspend it. Transit passage applies in international straits: it cannot be suspended, submarines may stay submerged, and aircraft have overflight rights. Transit passage is deliberately less restrictive to keep global chokepoints open (Wikipedia).
What rights does a country have in its EEZ?
Within its 200 nm EEZ, a coastal state has sovereign rights over all natural resources — fish, oil, gas, seabed minerals — and jurisdiction over marine research and environmental protection. Other nations retain full freedom of navigation and overflight; foreign warships can transit freely without permission, but cannot fish or conduct research without consent (Britannica).
Why has the US not ratified UNCLOS?
The US signed but never ratified UNCLOS — primarily due to Senate objections to Part XI’s deep seabed mining provisions, which created the International Seabed Authority to regulate resources the US wanted to exploit independently. Despite non-ratification, the US treats most UNCLOS provisions as customary international law and enforces navigation rights through Freedom of Navigation Operations (FONOPs) globally.
What is the 2023 High Seas Treaty?
The 2023 High Seas Treaty (formally the BBNJ Agreement) is a landmark instrument agreed in March 2023 under UNCLOS to protect ocean life in international waters. It introduces marine protected areas, environmental impact assessments, and benefit-sharing from marine genetic resources — the first major governance framework specifically for the high seas (Wikipedia).
UNCLOS isn’t just a legal document — it’s the framework within which every major naval standoff, fishing dispute, and resource competition plays out. Understanding it at this level, not just memorising zone distances, is what separates confident exam answers from guesswork.
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