Blockades are attempts by one belligerent state to obstruct a port or an entire coast of another state to prevent shipping or communication by sea with that state by allied or neutral powers. Blockades come in two categories, military and commercial. Military (or naval) blockades try to prevent an enemy's armed forces from leaving port, to bring them to action before they can carry out the mission for which they left port. Their ultimate purpose is to secure command of the sea, and their executors usually are battle squadrons. Commercial blockades have no immediate military objective but seek first and foremost to halt the enemy's seaborne trade by denying it the use of the sea-lanes of communications, ultimately forcing the adversary to surrender. Commercial blockades usually are conducted by cruisers, or in some cases, submarines. Blockades may encompass the whole enemy coast or part of it, and they often combine military and commercial blockades.
Blockades may also be "close" or "open." The former, as exercised by Britain when it bottled up French naval forces in ports and denied merchants access to seaborne shipping and communication during the rule of
Napoleon, are exhausting in terms of wear and tear to both ships and men; they demand a force greater than that against which they act and require large reserves for their relief. Close blockades became impractical with the development of mines and submarines—naval commanders declined to risk expensive capital ships to patrol lethal waters close to hostile shores. Open blockades, as executed by Britain against Germany from 1914 to 1918 and again from 1939 to 1945, strive to protect one's own trade and communications and deny command of home waters to the enemy. The penalty for blockade violators is usually loss of ship and cargo.
Modern common law has defined five conditions of blockade. First, it must be established by authority of a belligerent government. Second, under Article 4 of the Declaration of Paris (1856), it must be effective and not simply a "paper blockade"—that is, an enemy's shores must be actively patrolled at all times; a mere declaration of intent to blockade is unlawful. Third, it must be continuously maintained and impartially enforced against all vessels. Fourth, there must be some violation by egress or ingress before a neutral vessel may be seized (London Naval Conference, 1909). And fifth, a vessel about to be seized must have knowledge of the blockade.
In modern times, famous blockades include those by Britain against French and Spanish ports, 1803-1814; by the United States against the Confederacy, 1861-1865; by Britain against Germany, August 1914-June 1919; by German U-boats against Britain and its allied and associated powers, February-June 1915, March-April 1916, and, more rigorously, February 1917-October 1918; by Britain against Germany, November 1939-May 1945; by German U-boats against Britain and the Allies, 1939-1945; and by the republics of the Western Hemisphere against all actions "contrary to [their] security" in January 1942 (Rio de Janeiro Foreign Ministers' Declaration). Most effective among these blockades were those by the United States against the Confederacy in the 1860s and against Japan in
World War II. More recent U.S. naval blockades were maintained against Cuba in 1962 and Vietnam in 1965. The United Nations in 1966 authorized Britain to blockade Rhodesian oil—an example of an internationally authorized commercial blockade.
Perhaps most severe in human terms was the open blockade of Germany in
World War I by Britain and its allied and associated powers. According to the German Health Office in 1919, the "hunger blockade" and its attendant malnutrition resulted in 763,000 deaths, raised child mortality by 50 percent, increased tubercular-related deaths by 72 percent, and caused a 51 percent surge in women's mortality. The blockade led to rampant cases of rickets, influenza, dysentery, scurvy, keratomalacia, and hunger edema. It also destroyed civilian morale in Germany, thus helping the Allies achieve victory in 1918.
Julian Corbett, Some Principles of Maritime Strategy (1988).