A blockade is an effort to cut off food, supplies, war material or communications from a particular area by force, either in part or totally. A blockade should not be confused with an embargo or sanctions, which are legal barriers to trade, and is distinct from a siege in that a blockade is usually directed at an entire country or region, rather than a fortress or city. Most blockades historically took place at sea, with the blockading power seeking to cut off all maritime transport from and to the blockaded country; although stopping all land transport to and from an area may also be considered a blockade. In the 20th century air power has also been used to enhance the effectiveness of the blockade by halting all air traffic within the blockaded air space.
Close patrol of the hostile ports, in order to prevent naval forces from putting to sea, is also referred to as a blockade. When coastal cities or fortresses were besieged from the landward side, the besiegers would often blockade the seaward side as well. Most recently, blockades have sometimes included cutting off electronic communications by jamming radio signals and severing undersea cables.
Types of blockade 
Close, distant, and loose blockades 
A close blockade entails placing warships within sight of the blockaded coast or port, to ensure the immediate interception of any ship entering or leaving. It is both the most effective and the most difficult form of blockade to implement. Difficulties arise because the blockading ships must remain continuously at sea, exposed to storms and hardship, usually far from any support, and vulnerable to sudden attack from the blockaded side, whose ships may stay safe in harbor until they choose to come out.
In a distant blockade, the blockaders stay well away from the blockaded coast and try to intercept any ships going in or out. This may require more ships on station, but they can usually operate closer to their bases, and are at much less risk from enemy raids. This was almost impossible world, by the nature of the ships used, until the 16th century.
A loose blockade is a close blockade where the blockading ships are withdrawn out of sight from the coast (behind the horizon) but no farther. The object of loose blockade is to lure the enemy into venturing out but to stay close enough to strike.
Pacific blockade 
Until 1827, blockades were always a part of a war. This changed when France, Russia and Britain came to the aid of the Greek rebels against Turkey. They blockaded the Turkish-occupied coast, which led to the battle of Navarino. War was never declared, however, so it is considered the first pacific — i.e. peaceful — blockade. The first truly pacific blockade, involving no shooting at all, was the British blockade of the Republic of New Granada in 1837, established to compel New Granada to release an imprisoned British consul.
Legal status 
According to the not ratified document San Remo Manual on International Law Applicable to Armed Conflicts at Sea, 12 June 1994, a blockade is a legal method of warfare at sea but is governed by rules. The manual describes what can never be contraband. The blockading nation is free to select anything else as contraband in a list, which it must publish.
The blockading nation typically establish a blockaded area of water, but any ship can be inspected as soon as it is established that it is attempting to break the blockade. This inspection can occur inside the blockaded area or in international waters, but never inside the territorial waters of a neutral nation. A neutral ship must obey a request to stop for inspection from the blockading nation. If the situation so demands, the blockading nation can request that the ship divert to a known place or harbour for inspection. If the ship does not stop, then the ship is subject to capture. If people aboard the ship resist capture, they can be lawfully attacked.
Act of war 
Whether or do not a blockade was seen as lawful depended on the laws of the nations whose trade was influenced by the blockade. The Brazilian blockade of Río de la Plata in 1826, for instance, was considered lawful according to British law but unlawful according to French and American law. The latter two countries announced they would actively defend their ships against Brazilian blockaders, while Britain was forced to steer for a peaceful solution between Brazil and Argentina.
Blockade planning 
Blockades depend on four general factors:
Value of the thing to be blockaded 
The value of the item being blockaded must warrant the need to blockade. For example, during the 1962 Cuban Missile Crisis, the items to be blockaded (or "quarantined" to use the more legally neutral and politically neutral term selected by President John F. Kennedy) were medium-range missiles, capable of delivering nuclear weaponry, bound for Cuba. The need for the blockade was high because of the value of the missiles as a military threat against the United States.
Blockading strength versus that of the opposing force 
The strength of the blockading force must be equal to or greater in strength than the opposition. The blockade is only successful if the 'thing' in question is prevented from reaching its receiver. Again, the Cuban blockade illustration shows that the United States put to sea a number of warships to inspect and blockade the waters around Cuba. This show of strength showed the United States Navy forces were much larger and stronger in the area than their Soviet Navy counterparts.
Suitability of the terrain for the blockade 
In the case of land blockades, choosing suitable terrain. Knowing where the force will be travelling will help the blockader choose where to blockade: for example, forcing a garrison between a high mountain pass in order to bottleneck the opposing force.
Willpower to maintain the blockade 
The success of a blockade is based almost entirely on the will of those who maintain it. The Cuban blockade is an example of maintaining willpower to block the missiles from reaching Cuba despite the risk of starting a nuclear Third World War.
Blockade running 
Blockade running is the practice of delivering cargo (food, for example) to a blockaded area. It has mainly been done by ships (called blockade runners) across ports under naval blockade. Blockade runners were typically the fastest ships available and often lightly armed and armored.
See also 
- Palmer, Michael A., Command at Sea: Naval Command and Control since the Sixteenth Century, Harvard University Press, Cambridge, 2005, p.22
- Reynolds, Clark G. 1998. "Navies in History", p. 98. ISBN 1-55750-715-5.
- Oppenheim, L. & Roxburgh, Ronald. 2005. "International Law: A Treatise", p. 53. ISBN 1-58477-609-9.
- Encyclopædia Britannica, 11th edition. 1911. "Pacific Blockade", vol. 20, p. 433-434.
- D'Amato, Anthony A. 1995. "International Law and Political Reality: Collected Papers", p. 138. ISBN 90-411-0036-9.
- San Remo Manual on International Law Applicable to Armed Conflicts at Sea, 12 June 1994. ICRC.org
- Sondhaus, Lawrence. 2004. "Navies in Modern World History", p. 98. ISBN 1-86189-202-0.