A blockade works by cutting off food, supplies, war material and communications from a particular area by force, often to win a war. The Union Navy blockaded Confederate ports by ship and submarine throughout the Civil War.

Naval and Land Blockades

While most blockades are naval in nature, land blockades are also possible. In one of the earliest such instances, the Spartans blockaded Athens by land during the Peloponnesian War in the 4th century B.C. There are several types of naval blockades.

Close Blockade

In a close blockade, warships anchor in sight of a port city and intercept all ships leaving or entering, then use the threat of force to achieve surrender, according to a Naval Warfare article. Ships that create this blockade must remain at sea, where they are exposed to storms and vulnerable to attack.

Distance Blockade

A distant blockade is done for the same purpose as a close blockade but is safer for the blockading force because ships can stay a safer distance from the port yet can still intercept ships.

Loose Blockade

In a loose blockade, ships stay out of sight of a port to lure an enemy out before they strike. Vice admiral Horatio Nelson used a loose blockade in 1805 to lure Napoleon's fleet out of the port of Cadiz, resulting in the naval battle of Trafalgar that destroyed the French fleet.

Selective Blockades

Selective blockades are done to prevent particular items from entering a port. For instance, during the 1962 Cuban Missile Crisis, the U.S. naval blockade prevented medium-range missiles from being delivered to Cuba, according to the U.S. Navy website.

Source:

Undersea Warfare: The Birth of Undersea Warfare

Solar Navigator: Horatio Nelson and the Royal Navy

Naval History & Heritage: The Naval Quarantine of Cuba, 1962

More Information:

Naval Warfare: History