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.”[2]
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.