In simple terms

A naval blockade is a warring state sealing off an enemy coast or ports by force, so that ships cannot get in or out. It is a method of warfare rather than a trade measure, and that difference decides which rules apply to it.

A blockading navy stops ships from countries that are not in the war at all, which is why blockades create disputes far beyond the two sides fighting.

How it works

Blockade sits mostly in customary law rather than treaty law. No major modern treaty governs war at sea. The reference text is the San Remo Manual of 1994, non-binding guidance drawn up by government officials, the International Committee of the Red Cross and other experts to restate how the law of armed conflict applies at sea. [1]

Legal writers set out four conditions for a lawful blockade. It must be declared and notified, giving the start date, the geographical limits and the general scope of the measures used. It must be effective, backed by forces genuinely able to make access dangerous, so a paper blockade announced without that force does not qualify. It must be enforced impartially against the ships of every nation, allied, neutral and enemy alike. [2]

A separate route runs through the United Nations. Article 42 of the UN Charter lists blockade among the measures the Security Council may authorize to maintain international peace and security. [3] A Security Council blockade and one imposed by a warring state rest on different legal ground.

Why it matters

A blockade is one of the few military acts aimed squarely at an economy rather than an army. Closing a port halts fuel, food, medicine and exports at once, so the effects reach households and firms thousands of kilometers away through shipping and insurance costs.

The basic rules of international humanitarian law still apply at sea, including the principle of distinction between military objectives and civilians. [1] A blockade is therefore not a space outside the law, even where the detailed rules remain contested.

Where you will see it

• News reports about a shipping lane or chokepoint closing during a war
• Marine insurance notices warning vessels away from a declared area
• Security Council debates about measures authorized under Article 42 [3]
• Histories of the Falklands and Malvinas conflict, the Iran-Iraq war and the Gulf War, where blockades and exclusion zones were argued over [1]

Example

Shipowners rerouted away from the coast once the blockade was declared, because a blockade applies to neutral vessels as well as to the enemy fleet.

Often confused with

An embargo or a sanctions regime restricts what a government and its own citizens may trade. A blockade physically stops other countries’ ships from reaching a coast.

This entry explains a legal term. Laws differ by country and change over time. It is not legal advice.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 28, 2026Researched and drafted with AI assistanceReport an issue