BLOCKADE (Fr. blocus, Ger. Blokade), a term used in maritime warfare. Originally a blockade by sea was probably nothing more than the equivalent in maritime warfare of a blockade or siege on land in which the army investing the blockaded or besieged place is in actual physical possession of a zone through which it can prevent and forbid ingress and egress. An attempt to cross such a zone without the consent of the investing army would be an act of hostility against the besiegers. A maritime blockade, when it formed part of a siege, would obviously also be a close blockade, being part of the military cordon drawn round the besieged place. Even from the first, however, differences would begin to grow up in the conditions arising out of the operations on land and on sea. Thus whereas conveying merchandise across military lines would be a deliberate act of hostility against the investing force, a neutral ship which had sailed in ignorance of the blockade for the blockaded place might in good faith cross the blockade line without committing a hostile act against the investing force. With the development of recognition of neutral rights the involuntary character of the breach would be taken into account, and notice to neutral states and to approaching vessels would come into use. With the employment in warfare of larger vessels in the place of the more numerous small ones of an earlier age, notice, moreover, would tend to take the place of de facto investment, and at a time when communication between governments was still slow and precarious, such notice would sometimes be given as a possible measure of belligerent tactics before the blockade could be actually carried out. Out of these circumstances grew up the abuse of "paper blockades."
The climax was reached in the "Continental Blockade" decreed by Napoleon in 1806, which continued till it was abolished by international agreement in 1812. This blockade forbade all countries under French dominion or allied with France to have any communication with Great Britain. Great Britain replied in 1807 by a similar measure. The first nation to protest against these fictitious blockades was the United States. Already in 1800 John Marshall, secretary of state, wrote to the American minister in Great Britain pointing out objections which have since been universally admitted. In the following interesting passage he said: -
"Ports not effectually blockaded by a force capable of completely investing them have yet been declared in a state of blockade.... If the effectiveness of the blockade be dispensed with, then every port of the belligerent powers may at all times be declared in that state, and the commerce of neutrals be thereby subjected to universal capture. But if this principle be strictly adhered to, the capacity to blockade will be limited by the naval force of the belligerent and, in consequence, the mischief to neutral commerce cannot be very extensive. It is, therefore, of the last importance to neutrals that this principle be maintained unimpaired. I observe that you have pressed this reasoning on the British minister, who replies that an occasional absence of a fleet from a blockaded port ought not to change the state of the place. Whatever force this observation may be entitled to, where that occasional absence has been produced by an accident, as a storm, which for a moment blows off a fleet and forces it from its station, which station it immediately resumes, I am persuaded that where a part of the fleet is applied, though only for a time, to other objects or comes into port, the very principle requiring an effective blockade, which is that the mischief can only be coextensive with the naval force of the belligerent, requires that during such temporary absence the commerce to the neutrals to the place should be free." 
Again in 1803 James Madison wrote to the then American minister in London: -
"The law of nations requires to constitute a blockade that there should be the presence and position of a force rendering access to the prohibited place manifestly difficult and dangerous." 
In 1826 and 1827 Great Britain as well as the United States asserted that blockades in order to be binding must be effective. This became gradually the recognized view, and when in 1856 the powers represented at the congress of Paris inserted in the declaration there adopted that "blockades in order to be binding must be effective, that is to say, maintained by a force sufficient really to prevent access to the coast of an enemy," they were merely enunciating a rule which neutral states had already become too powerful to allow belligerents to disregard.
Blockade is universally admitted to be a belligerent right to which under international law neutrals are obliged to submit. It is now also universally admitted that the above-quoted rule of the Declaration of Paris forms part of international law, independently of the declaration. Being, however, exclusively a belligerent right, it cannot be exercised except by a belligerent force. Even a de facto belligerent has the right to institute a blockade binding on neutrals if it has the means of making it effective, though the force opposed to it may treat the de facto belligerent as rebels.
It is also admitted that, being exclusively a belligerent right, it cannot be exercised in time of peace, but there has been some inconsistency in practice (see Pacific Blockade) which will probably lead governments, in order to avoid protests of neutral powers against belligerent rights being exercised in mere coercive proceedings, to exercise all the rights of belligerents and carry on de facto war to entitle them to use violence against neutral infringers. This was done in the case of the blockade of Venezuela by Great Britain, Germany and Italy in 1902-1903.
The points upon which controversy still arises are as to what constitutes an "effective" blockade and what a sufficient notice of blockade to warrant the penalties of violation, viz. confiscation of the ship and of the cargo unless the evidence demonstrates the innocence of the cargo owners. A blockade to be effective must be maintained by a sufficient force to prevent the entrance of neutral vessels into the blockaded port or ports, and it must be duly proclaimed. Subject to these principles being complied with, "the question of the legitimacy and effectiveness of a blockade is one of fact to be determined in each case upon the evidence presented" (Thomas F. Bayard, American secretary of state, to Messrs Kamer & Co., 19th of February 1889). The British manual of naval prize law sums up the cases in which a blockade, validly instituted, ceases to be effectively maintained, as follows: - (1) If the blockading force abandons its position, unless the abandonment be merely temporary or caused by stress of weather, or (2) if it be driven away by the enemy, or (3) if it be negligent in its duties, or (4) if it be partial in the execution of its duties towards one ship rather than another, or towards the ships of one nation rather than those of another. These cases, however, are based on decisions of the British admiralty court and cannot be relied on absolutely as a statement of international law.
As regards notice the following American instructions vere given to blockading officers in June 1898: -
"Neutral vessels are entitled to notification of a blockade before they can be made prize for its attempted violation. The character of this notification is not material. It may be actual, as by a vessel of the blockading force, or constructive, as by a proclamation of the government maintaining the blockade, or by common notoriety. If a neutral vessel can be shown to have had notice of the blockade in any way, she is good prize, and should be sent in for adjudication; but should formal notice not have been given, the rule of constructive knowledge arising from notoriety should be construed in a manner liberal to the neutral.
"Vessels appearing before a blockaded port, having sailed without notification, are entitled to actual notice by a blockading vessel. They should be boarded by an officer, who should enter in the ship's log the fact of such notice, such entry to include the name of the blockading vessel giving notice, the extent of the blockade, the date and place, verified by his official signature. The vessel is then to be set free; and should she again attempt to enter the same or any other blockaded port as to which she has had notice, she is good prize. Should it appear from a vessel's clearance that she sailed after notice of blockade had been communicated to the country of her port of departure, or after the fact of blockade had, by a fair presumption, become commonly known at that port, she should be sent in as a prize."
The passages in italics are not in accordance with the views held by other states, which do not recognize the binding character of a diplomatic notification or of constructive notice from notoriety.
The subject was brought up at the second Hague Conference (1907). The Italian and Mexican delegations submitted projects, but after a declaration by the British delegate in charge of the subject (Sir E. Satow) that blockade not having been included in the Russian programme, his government had given him no instructions upon it, the subject, at his suggestion, was dropped. A Voeu, however, was adopted in favour of formulating rules on all branches of the laws and customs of naval war, and a convention was agreed to for the establishment of an international Prize Court (see Prize). Under Art. 7 of the latter convention the Court was to apply the "rules of international law," and in their absence the "general principles of justice and equity." As soon as possible after the close of the second Hague Conference the British government took steps to call a special conference of the maritime powers, which sat from December 4, 1908 to February 26, 1909. Among the subjects dealt with was Blockade, the rules relating to which are as follow: -
Art. 1. A blockade must not extend beyond the ports and coasts belonging to or occupied by the enemy.
Art. 2. In accordance with the Declaration of Paris of 1856, a blockade, in order to be binding, must be effective - that is to say, it must be maintained by a force sufficient really to prevent access to the enemy coastline.
Art. 3. The question whether a blockade is effective is a question of fact.
Art. 4. A blockade is not regarded as raised if the blockading force is temporarily withdrawn on account of stress of weather.
Art. 5. A blockade must be applied impartially to the ships of all nations.
Art. 6. The commander of a blockading force may give permission to a warship to enter, and subsequently to leave, a blockaded port.
Art. 7. In circumstances of distress, acknowledged by an officer of the blockading force, a neutral vessel may enter a place under blockade and subsequently leave it, provided that she has neither discharged nor shipped any cargo there.
Art. 8. A blockade, in order to be binding, must be declared in accordance with Article 9, and notified in accordance with Articles 11 and 16.
Art. 9. A declaration of blockade is made either by the blockading power or by the naval authorities acting in its name. It specifies (1) the date when the blockade begins; (2) the geographical limits of the coastline under blockade; (3) the period within which neutral vessels may come out.
Art. 10. If the operations of the blockading power, or of the naval authorities acting in its name, do not tally with the particulars, which, in accordance with Article 9 (1) and (2), must be inserted in the declaration of blockade, the declaration is void, and a new declaration is necessary in order to make the blockade operative.
Art. 11. A declaration of blockade is notified: (1) to neutral powers, by the blockading power by means of a communication addressed to the governments direct, or to their representatives accredited to it; (2) to the local authorities, by the officer commanding the blockading force. The local authorities will, in turn, inform the foreign consular officers at the port or on the coastline under blockade as soon as possible.
Art. 12. The rules as to declaration and notification of blockade apply to cases where the limits of a blockade are extended, or where a blockade is re-established after having been raised.
Art. 13. The voluntary raising of a blockade, as also any restriction in the limits of a blockade, must be notified in the manner prescribed by Article 11.
Art. 14. The liability of a neutral vessel to capture for breach of blockade is contingent on her knowledge, actual or presumptive, of the blockade.
Art. 15. Failing proof to the contrary, knowledge of the blockade is presumed if the vessel left a neutral port subsequently to the notification of the blockade to the power to which such port belongs, provided that such notification was made in sufficient time.
Art. 16. If a vessel approaching a blockaded port has no knowledge, actual or presumptive, of the blockade, the notification must be made to the vessel itself by an officer of one of the ships of the blockading force. This notification should be entered in the vessel's logbook, and must state the day and hour, and the geographical position of the vessel at the time. If through the negligence of the officer commanding the blockading force no declaration of blockade has been notified to the local authorities, or if in the declaration, as notified, no period has been mentioned within which neutral vessels may come out, a neutral vessel coming out of the blockaded port must be allowed to pass free.
Art. 17. Neutral vessels may not be captured for breach of blockade except within the area of operations of the warships detailed to render the blockade effective.
Art. 18. The blockading forces must not bar access to neutral ports or coasts.
Art. 19. Whatever may be the ulterior destination of a vessel or of her cargo, she cannot be captured for breach of blockade, if, at the moment, she is on her way to a non-blockaded port.
Art. 20. A vessel which has broken blockade outwards, or which has attempted to break blockade inwards, is liable to capture so long as she is pursued by a ship of the blockading force. If the pursuit is abandoned, or if the blockade is raised, her capture can no longer be effected.
Art. 21. A vessel found guilty of breach of blockade is liable to condemnation. The cargo is also condemned, unless it is proved that at the time of the shipment of the goods the shipper neither knew nor could have known of the intention to break the blockade.
 John Marshall, secretary of state, to Rufus King, minister to England, 20th of September 1800, Am. State Papers, Class I, For. Rel. II, No. 181, J.B. Moore, Digest of International Law, vii. 788.
 James Madison, secretary of state, to Mr Thornton, 27th of October 1803, 14 MS. Dom. Let. 215. Moore, Digest of International Law, vii. 789.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)