Blockade

Blockade, naval operation conducted by a country at war, with the object of closing to foreign commerce the vital ports of an enemy country and thereby aiding in the military defeat of that country by denying it access to supplies and communications from without. In 1805, during the Napoleonic Wars, France declared a blockade against the British Isles. Britain retaliated by declaring a blockade against all continental Europe. Neither of the warring countries, however, possessed navies large enough to enforce the blockades. Such blockades, known as paper blockades, were merely excuses for capturing neutral ships. It is now a principle of international law that a valid blockade (a blockade is a legal act of war) must fulfill the following conditions: (1) it must be ordered by the sovereign or war-making authority of a belligerent power; (2) it must be effective; and (3) neutral countries must be notified of the blockade and their vessels given reasonable time to leave the waters of the blockaded country. If a neutral ship attempts to break the blockade after such time has elapsed, ship and cargo may lawfully be seized, but no punishment may be inflicted upon the crew. Neutral warships on diplomatic errands and ships in distress are generally allowed entry to a blockaded port.

The most effective blockade of a long coast was that maintained by the U.S. Navy during the American Civil War. Northern warships prevented commerce between Britain, in need of southern cotton, and the South, in need of British-manufactured articles. Only a few fast-sailing vessels, known as blockade-runners, managed at times to slip through.

During World War I the Central Powers were blockaded by the navies of the Allies. Although the blockade was not altogether successful in the Baltic Sea, it cut Germany off from the Atlantic Ocean almost entirely. Germany failed in its attempt to break the blockade by the use of submarines. Its declaration of a submarine blockade against Britain was regarded as invalid on the ground that the German submarine fleet alone was not strong enough to maintain such a blockade effectively. Neutral countries asserted that it was a paper blockade and that Germany, therefore, had no lawful right to sink or seize neutral ships headed for British ports. German sinking of American ships despite repeated protests was one cause of the U.S. entry into World War I.

Blockade operations by the U.S. and Britain during World War II were an essential part of military strategy, supplementing air raids, which struck directly at the industrial economy of the enemy. During the Korean War, United Nations naval forces blockaded the Korean coast.

When a blockade is imposed by a power in order to coerce another power, without resorting to war, the blockade is termed a pacific blockade. The U.S. enforced this type of blockade on Cuba in 1962. See also Embargo.