Blockade, in international law, the closing of an enemy's port by a besieging force. It has been described by Sir William Scott as "a sort of circumvallation round a place, by which all foreign connection and correspondence is, as far as human power can effect it, to be entirely cut off." The circumstances essential to a valid blockade are tolerably well settled by the decisions of eminent jurists in prize cases. The first of these is that a state of war must exist, though this may be without an actual declaration of war, for the blockade may be the first hostile act. The second is that it be sustained by a blockading force sufficient to make it hazardous to attempt to enter or depart from the port, although if the ships composing it be for any short time driven from their positions by sudden tempest or other similar cause, the blockade is not thereby raised. The purpose of this measure is to inflict injury upon an enemy, either by reducing the place, or by weakening his power of resistance by cutting off his supplies, or both; but as a considerable proportion of the injury must fall upon neutrals, the belligerent is justly required to make his blockade what the term imports, and neither would neutral nations submit to it if he did not, nor would the prize courts sanction the captures which might be made for evading it.

The third circumstance essential is that a neutral against whom it is sought to be enforced should have been notified of it. The notice may be by formal notification of the executive published to the world, or actual notice at the time trade with the port is attempted; but notice may be presumed in any case where the blockade has become matter of public and general notoriety. The privilege of the blockading force is to seize and send in for condemnation any vessel with its cargo endeavoring to trade with the port; and if the vessel succeeds in violating it, she may be followed and seized on the high seas, and does not purge herself of the offence until she has returned to the port from which she originally set out. In cases of neutral vessels in port when the blockade is declared, the notoriety of the act is sufficient notice; they are at liberty to leave with such cargo as they may then have on board, but must not take on more. A neutral vessel incurs no liability in trading at a port not blockaded in goods destined to the blockaded port by land carriage. - Some notable attempts have been made to enforce mere paper blockades.

The Berlin decree of Nov. 21, 1806, of the emperor Napoleon, declared all the British islands in a state of blockade, and threatened capture and condemnation to vessels trading with them. The English government retaliated, and between the Berlin and Milan decrees on the one hand and the orders in council on the other, though no actual blockade was established, all neutral trade with Great Britain and France and their respective colonies and dependencies was threatened with destruction. The United States was the principal sufferer from these measures, and justly considered herself entitled to redress. The breaking out of the civil war in the United States in 1861 presented some embarrassing questions as to the proper course to take in regard to the southern ports. Two courses were open to the government: to declare the ports closed as ports of entry, or to establish a blockade. As the ports belonged to the country, and it was the right of the government to declare what should and what should not be ports of entry, it was argued by some that the simplest course to take was to exercise the undoubted right to close them, and thereby render all trade with them unlawful.

Such a course, however, it must be evident, would be taken not in the interests of commerce and not for any motive operating in time of peace, and therefore, whatever name might be given it, would be really a belligerent act resorted to in order to inflict injury upon a public enemy; and it was highly probable that neutral nations would insist that, though called a mere municipal regulation, it was in its nature an attempt at blockade, and to be respected must appear to be made by the proper force. The government took the other course, and in April, 1861, the president issued proclamations declaring the southern ports blockaded. The blockade at first was not so complete as afterward, and some vigorous remonstrances were made against it in England as being in law wholly ineffectual; but the British government, after careful investigation, did not venture to pronounce it insufficient, and correctly laid down the rule of law as follows: "Her majesty's government are of opinion that, assuming the blockade is duly notified, and also that a number of ships is stationed and remains at the entrance of a port sufficient really to prevent act ess to it, or to create an evident danger in entering or leaving it, and that these ships do not voluntarily permit ingress or egress, the fact that various ships may have successfully escaped through it will not of itself prevent the blockade from being an effective one by international law." Notwithstanding a considerable trade was carried on through the blockaded ports by means of swift vessels constructed for the purpose, this conclusion of the British government was adhered to; the prize courts declared the same doctrine, and Secretary Welles in his annual report for the second year of the war was able to boast of the blockade as "the greatest of all naval triumphs." But some of the ports it was found impossible wholly to close, and in a few instances, notably in the case of Charleston, an attempt was made to preclude passage through some of the channels by sinking therein old vessels, stones, and other obstructions.

This, being taken as an attempt to destroy the ports, was remonstrated against by the British minister, as not sanctioned by the laws of war; but it was replied by Mr. Seward that the obstructions were only temporary, and in fact they proved of little importance. - A blockade terminated is said to be raised, and this may be done by public proclamation or by withdrawing the blockading force.