Blockade, in a military sense, is an operation for capturing an enemy's town or fortress, often without a bombardment or regular siege. The attacking party throws up works on the neighbouring heights and roads, so as to guard every exit from the town. The rest of the besieging force remains under cover in villages, or in a temporary camp, ready to repel any sortie attempted by the besieged. The whole purpose in view is to prevent the besieged from receiving supplies of any kind, in order that, when the food or the ammunition is exhausted, they may be compelled to surrender. Fortresses situated on steep and rocky eminences, difficult to conquer by bombardment or assault, may often be reduced by blockade; because the roads or paths for the reception of supplies are few, and can be watched by a small number of troops. Towns situated on a plain are less easily invested; but if the inhabitants are numerous and commercial, they will soon be impatient of the restraint produced by a blockade, and compel the governor to surrender. When, therefore, resistance is determined on, the governor sends out of the town as many non-combatants as possible. He then collects all the stores in bomb-proofs, places the inhabitants on an allowance of food and under military rule, and endeavours, by frequent sorties, to prevent the besiegers from completing the investment of the place. A blockade should be the first step in a regular siege, and its effects may often be made more quickly felt by a bombardment. The blockade of Paris by the Germans in 1870-71 is the most notable example of such an operation that has ever been known. It was of itself completely successful, and the bombardment which was thought necessary might have been dispensed with. In this case a large civil population was blockaded together with a numerous army; but at Metz, in the same campaign, a large and well appointed army, comparatively unhampered by the civil population, was compelled to surrender by a blockade without bombardment (see also SIEGE). Blockade on land depends on sovereignty or on military occupation. But at sea the rights of the neutral are prima facie equal to those of the belligerents.
Blockading, in a naval sense, is the prevention of the entrance or exit of the enemy's ships at a particular port, or at all the ports on a stretch of coast, so as to bring pressure to bear upon the inhabitants by obstructing their trade; and it renders intercourse with the enemy's ports unlawful on the part of neutrals. It is also sometimes an auxiliary to the military blockade by land. For a valid blockade it is necessary that a state of war should exist; that the blockade be really effective, that is to say, it must be maintained by a force sufficient to prevent access to the enemy's port, or at least to render approach hazardous; and neutral nations must be informed by the blockading power, either by official notification to all those powers, or by warning given to neutral vessels which approach the line of blockade. But when a blockade has become notorious, the knowledge will be presumed against any merchant-vessel which attempts to enter the blockaded port. Effective blockade is the principle laid down in the Declaration of Paris (1856), but this is construed more rigidly by continental nations than by England or the United States. Riga was effectively blockaded during the Crimean war at a distance of 120 miles. The French insist more than we do on formal notice. The blockading force may seize any vessel with its cargo trying to trade with the port, and send it home for condemnation; and if the vessel succeed in breaking the blockade, it may be pursued and captured by a belligerent, until it has reached its port of destination. That breach of blockade involves confiscation of the ship is admitted by all civilised nations. The cargo is not forfeited unless its owner is the owner of the ship, or was cognisant of the intended violation.
Napoleon's Berlin decree of 1806 declared the British Islands in a state of blockade (see CONTINENTAL SYSTEM).
The most memorable of recent blockades is that of the ports of the Southern States by the Federal government during the American civil war. The blockade was begun in April 1861, and, extending as it did to the whole of the southern coast and ports, was at first somewhat ineffective. Ultimately it was more rigorous and systematic; but to the last a considerable trade was carried on by swift steamers (many of them Clyde-built) through such blockaded ports as Charleston (the harbour of which was obstructed by sinking old ships and stones to assist the blockaders) and Wilmington. The risks were great. Hobart Pasha, who was a noted blockade-runner, says that more than 40 ships were captured out of 66 that left England and New York to run the blockade during the four years of the war. But the profits were enormous; Hobart sold in Wilmington for twelve shillings women's stays which he bought at thirteenpence, and sold cotton in Liverpool for half-a-crown which in Wilmington cost him only twopence a pound. Besides those captured, many blockade-runners had to be run ashore or burnt to escape the cruisers. Nassau, in the Bahamas, was a great station for ships in the blockade-running enterprise. See CHARLESTON, NEUTRALITY, PRIZE COURT; Hobart Pasha's Sketches from my Life (1886); T. S. Taylor's Running the Blockade (1896); and as to the law, Twiss, Hall, Phillimore, Hazlitt, and Roche for England; Wheaton for the United States; Hautefeuille, Heffer, Gessner, and
Bluntschli for the continent of Europe; the collections of treaties; and the proceedings of the Alabama Commission. Lord Stowell's decisions in the prize court during the French wars may be said to form the substance of English law on this subject.