What a naval blockade is — why declaring one is not enough
A naval blockade is the cutting off of maritime traffic to and from enemy coasts or ports, and international law treats it as an act of war. The framework runs from the 1856 Paris Declaration through the 1909 London Declaration to the 1994 San Remo Manual. Its oldest requirement is effectiveness — a blockade that is only declared, with no force able to enforce it, is not recognised as a blockade
The three lines
- Definition — cutting off maritime access to a coast or ports. In law, an act of war
- Requirements — declaration, notification, impartial application, and effectiveness. A paper blockade is not a blockade
- Framework — the 1856 Paris Declaration, the 1909 London Declaration, and the 1994 San Remo Manual
Key questions
- What counts as a naval blockade?
- The **physical cutting off of maritime traffic** to and from an enemy's coast or ports. What distinguishes it from sanctions or an embargo is that the object is **passage itself**, not particular cargo or particular vessels. Economic sanctions are domestic-law instruments — a state ordering its own firms and nationals not to trade. A blockade **physically stops ships, including third-country ships**. For that reason international law classifies it not as a trade measure but as an **act of war**: declaring a blockade is understood to place the declaring state in a state of armed conflict.
- Is declaring a blockade sufficient to establish one?
- No — and this is the oldest rule in the field. The **1856 Paris Declaration** states that blockades, in order to be binding, **must be effective**. Its target was the practice known as the **paper blockade**: declaring a coast closed without stationing enough ships to enforce it, then seizing whichever neutral merchantmen happened by. That practice loaded unpredictable risk onto neutral shipping. Since Paris, whether a blockade exists has been a **question of fact** — is there a force actually capable of denying access? — and a declaration that fails that test is not a blockade in law.
- What makes a blockade lawful?
- Custom and treaty converge on four requirements. ① **Declaration** — the start date, duration, geographic extent, and the period in which neutral vessels may leave must be stated. ② **Notification** — belligerents and neutrals must be informed. ③ **Effectiveness** — access must actually be deniable. ④ **Impartiality** — the blockade must apply equally to the vessels of all states; letting one country's ships through breaks the requirement. The modern **1994 San Remo Manual** adds humanitarian limits: a blockade whose sole or primary purpose is to starve the civilian population is prohibited, and passage must be allowed for humanitarian relief.
Through 2026 the same word has run through coverage of the Strait of Hormuz.
Blockade
But the everyday word and the legal term are different. In ordinary use it means "closed off." In international law it means a specific measure meeting specific requirements — and a measure that fails them is not a blockade at all.
The oldest of those requirements is this one:
Declaring it is not enough.
1. How it differs from sanctions and embargoes
| Measure | Object | Character | Third-country ships |
|---|---|---|---|
| Economic sanctions | Own firms and nationals | Domestic-law order | Not directly compelled |
| Embargo | Trade in specified goods | Domestic law, multilateral agreement | Not directly compelled |
| Naval blockade | Passage itself | Act of war | Physically stopped |
The final column is the distinction that matters.
Sanctions instruct a state's own firms not to trade with a target. They do not directly bind third-country firms — reaching those requires a separate device such as a secondary boycott, whose structure this page examined on August 23.
A blockade is different. It physically stops any ship, of any flag. That is why international law classifies it not as a trade measure but as an act of war: to declare a blockade is understood to enter a state of armed conflict.
That classification has a practical edge. Recognising a measure as a blockade opens space for the other side's use of force to be legally justified. Which is why states that are in practice stopping traffic will often avoid calling it a blockade.
2. The line of instruments — 170 years
| Year | Instrument | Contribution |
|---|---|---|
| 1856 | Declaration of Paris | Effectiveness requirement; abolition of privateering |
| 1909 | Declaration of London | 21 articles on blockade; procedural rules |
| 1994 | San Remo Manual | Modern restatement; humanitarian limits |
The 1909 London Declaration never entered into force — most signatories did not ratify. But much of it transcribed existing custom, so it has continued to be cited in practice.
The 1994 San Remo Manual is not a treaty either. It is a text produced by a group of experts. It is nonetheless widely used as the reference in naval manuals and legal practice. Which provisions a given state accepts as customary law varies.
3. The four requirements
Custom and text converge on four.
| Requirement | Content |
|---|---|
| Declaration | State the start date, duration, geographic extent, and the period for neutral vessels to leave |
| Notification | Inform belligerents and neutrals |
| Effectiveness | Access must actually be deniable |
| Impartiality | Apply equally to the vessels of all states |
The third and fourth are the ones that generate disputes.
4. Effectiveness — a rule written against paper blockades
The Paris Declaration's sentence is short.
"Blockades, in order to be binding, must be effective."
It was aimed at a specific practice: the paper blockade.
Maritime powers would declare a stretch of coast blockaded without deploying ships sufficient to enforce it — then seize neutral merchantmen that passed through. The declaration itself served as the legal basis for the seizure.
The problem lay with neutrals. They could not know where the blockade actually extended, or whether it was being enforced at all. Unquantifiable risk suppressed shipping generally.
Paris inverted the structure. It made the existence of a blockade a matter of fact rather than proclamation.
| Before Paris | After | |
|---|---|---|
| Basis of existence | The declaration | Actual deployment |
| Who decides | The blockading power | A finding of fact |
| Neutral risk | Unpredictable | Ascertainable |
The San Remo Manual puts it in one line — whether a blockade is effective is a question of fact.
5. Impartiality — exceptions dissolve the requirement
The fourth requirement does heavy work in practice.
A blockade must apply equally to the ships of all states. Letting one country's vessels through breaks it.
The logic runs like this. What justifies a blockade as an act of war is military necessity. If some flags pass and others do not, the measure is no longer serving military necessity — it is selective economic pressure, and it loses its character as a blockade in law.
Which raises the hard contemporary question. What is a traffic restriction run as a system of selective permissions? It fails the requirements of a blockade, yet its practical effect is a blockade's.
This page described such a case on August 24 in Iran lets some Iraqi tankers through Hormuz on August 22, 2026. Whether that measure legally constitutes a blockade is not for this page to determine — that requires established facts and a competent decision-maker.
6. The humanitarian layer, added in 1994
The San Remo Manual added constraints that the nineteenth-century instruments did not carry.
| Limit | Content |
|---|---|
| Purpose | A blockade whose sole or primary purpose is to starve the civilian population is prohibited |
| Proportionality | Prohibited where civilian harm is excessive relative to the anticipated military advantage |
| Passage | Humanitarian relief and essential medical supplies must be allowed through |
These emerged from the blockade record of the twentieth century's two world wars, in which the instrument was used against civilian populations rather than armies.
7. Why this body of law is being read again
The law of blockade was for a long time close to a historical subject, since full naval war between states was rare after 1945.
It is being searched again in 2026 for two reasons.
First, economic coercion has moved to sea. Financial sanctions dilute through workaround routes; physically closing a corridor does not.
Second, energy routes are geographically narrow. A large share of the world's seaborne oil passes through a handful of straits. A narrow corridor is also one where the effectiveness requirement can be satisfied with modest forces.
This page described that geography on August 3 in The Strait of Hormuz — 34km carrying a fifth of the world's oil. The legal significance of that 34km is here.
8. What is not confirmed
- Application to current events — this is a statement of the general law; it does not determine whether any 2026 measure in Hormuz legally constitutes a blockade.
- Status of San Remo — an expert text, not a treaty. States differ on which provisions are customary.
- The London Declaration — never entered into force; treated only as reflecting custom.
- Relation to the UN Charter — the interaction with Article 42 measures is contested and not addressed here.
- Next checkpoint — whether selective permission continues. The gap between the impartiality requirement and actual practice is what determines this situation's legal character.
Sources
- Wikipedia — Paris Declaration Respecting Maritime Law (1856)
- International Institute of Humanitarian Law — San Remo Manual on International Law Applicable to Armed Conflicts at Sea (1994)
- US Naval War College — Revising the San Remo Manual Provisions on Blockade
- Oxford Public International Law (MPEPIL) — Blockade
- The Practical Guide to Humanitarian Law — Blockade
- International Review of the Red Cross — The ever-existing 'crisis' of the law of naval warfare
- Lieber Institute, West Point — The San Remo and the Newport Manuals on the Law of Naval Warfare