A blockade is an action by a naval power that denies most shipping, and all military vessels, from access to the ports and coast of a state under blockade. It must be formally declared, and the "declaration shall specify the commencement, duration, location, and extent of the blockade and the period within which vessels of neutral States may leave the blockaded coastline." While it is defined in terms of the sea by international law, the term has been used for other operations of denial, such as the Berlin Blockade. It is also applicable to quasi-states or states in rebellion, as in the Union Blockade during the American Civil War.
To be recognized in international law, it must be effective. "The question whether a blockade is effective is a question of fact. The force maintaining the blockade may be stationed at a distance determined by military requirements."
"Merchant vessels believed on reasonable grounds to be breaching a blockade may be captured. Merchant vessels which, after prior warning, clearly resist capture may be attacked. A blockade must not bar access to the ports and coasts of neutral States.
"The declaration or establishment of a blockade is prohibited if:
- it has the sole purpose of starving the civilian population or denying it other objects essential for its survival; or
- the damage to the civilian population is, or may be expected to be, excessive in relation to the concrete
and direct military advantage anticipated from the blockade.
"If the civilian population of the blockaded territory is inadequately provided with food and other objects essential for its survival, the blockading party must provide for free passage of such foodstuffs and other essential supplies, subject to:
- the right to prescribe the technical arrangements, including search, under which such passage is permitted; and
- the condition that the distribution of such supplies shall be made under the local supervision of a Protecting Power or a humanitarian organization which offers guarantees of impartiality, such as the International Committee of the Red Cross.
"The blockading belligerent shall allow the passage of medical supplies for the civilian population or for the wounded and sick members of armed forces, subject to the right to prescribe technical arrangements, including search, under which such passage is permitted.
- "San Remo Manual on International Law Applicable to Armed Conflicts at Sea", International Review of the Red Cross (no. 309): 595-637, 31 December 1995, Articles 93-104