BLOCKADE. In International Law. The actual investment of a port or place by a hostile force fully competent, under ordinary circumstances, to cut off all communication therewith, so arranged or disposed as to be able to apply its force to every point of practicable access or approach to the port or place so invested.
Nature and character. Blockades may be either military or commercial, or may par take of the nature of both. As military blockades they may partake of the nature of a land or land and sea investment of a besieged city or seaport, or they may con sist of a masking of the enemy's fleet by another belligerent fleet in a port or anchor age where commerce does not exist. As commercial blockades, they may consist of operations against an enemy's trade or reve nue, either localized at a single important seaport, or as a more comprehensive strat egic operation, by which the entire sea fron tier of an enemy is placed under blockade. A blockade, being an operation of war, any government, independent or de facto, whose rights as a belligerent are recognized, can institute a blockade as an exercise of those rights.
The justification of blockade lies In the international recognition of the necessity which the belligerent is under of imposing that restriction upon neutral commerce for the successful prosecution of hostilities. It is not settled whether the mouth of an international river can be blockaded in case one or more of the upper riparian states re main neutral. But if a river constitutes the boundary line between a belligerent and a neutral, it may not be blockaded so as to prevent access to the neutral side of the river. The Peterhoff, 5 Wall. (U. S.) 49, 18 L. Ed. 564. In case of civil war, a govern ment may blockade certain of its own ports, as was done by the United States during the American Civil War and by France during the Franco-Prussian War.
Effectiveness. In international jurispru dence it is a well-settled principle that the blockading force must be present and of suf ficient force to be effective, and a mere no tification of one belligerent that the port of the other is blockaded, sometimes termed paper blockade, is not sufficient to establish a legal blockade. A blockade may be made
effective by batteries on shore as well as by ships afloat, and, in case of inland ports, may be maintained by batteries command ing the river or inlet by which it may be approached, supported by a naval force suf ficient to warn off innocent and capture of fending vessels attempting to enter; The Cir cassian, 2 Wall. (U. S.) 135, 17 L. Ed. 796. In 1856 the Declaration of Paris prescribed that blockades to be obligatory must be ef fective, that is to say, maintained by a suf ficient force really to prevent access of the enemy's ships and other vessels. The United States, although not a party to this declara tion, has upheld the same doctrine since 1781, when, by ordinance of Congress, it was declared that there should be a number of vessels stationed near enough to the port to make the entry apparently dangerous; Jour nals of Congress, vol. vii. p. 186. By the convention of the Baltic Powers in 1780, and again in 1801, the same doctrine was pro, mulgated; and in 1871, by treaty between Italy and the United States, a clearer and more satisfactory definition of an effective blockade was agreed upon, as follows: "It is expressly declared that such places only shall be considered blockaded as shall be actually invested by naval forces capable of pnventing the entrance of neutrals, and so stationed as to create an evident danger on their part to attempt it." The French doctrine of an effective block ade is that access must be barred by a line of ships forming a chain around the block aded port, while the United States, Great Britain and Japan hold that it is sufficient to have men-of-war cruising in the vicinity of the port, provided the disposition of the cruisers constitutes an actual danger to a vessel seeking to run the blockade. A block ade does not cease to be effective because the blockading force is temporarily with drawn owing to stress of weather. 1 C. Rob. 86, 154. If a single modern cruiser, blockad ing a port, renders it in fact dangerous for other craft to enter the port, the blockade is practically effective ; the Olinde Rodrigues, 174 U. S. 510, 19 Sup. Ct. 851, 43 L. Ed. 1065.