Nelson hid his ships over the horizon to lure out a fleet
A blockade does not aim to capture a city; it aims to choke a whole country. Britain perfected the full naval version against France in the 1750s, and in 1805 Nelson pulled his ships just out of sight at Cádiz, tempting the enemy fleet to sail. The result was Trafalgar.
A blockade uses armed force to stop goods, weapons, messages and sometimes people moving in or out of a country or region. That makes it different from an embargo or sanctions, which are legal restrictions, and from a siege, which targets one fortified place in order to reduce it. Neutral ships caught up in a blockade must submit to searches for contraband, a category the blockader may define broadly. In the 20th century, aircraft were added to shut down air traffic, and modern versions have jammed radio and cut undersea cables.
Ancient navies tried blockades, but ships could not stay at sea long enough to keep one up. The Royal Navy made the first successful full blockade during the Seven Years' War. After the 1759 victory at Quiberon Bay, Admiral Edward Hawke stretched a cordon along France's Atlantic coast from Dunkirk to Bordeaux and on to Marseille, strangling trade. Blockades also starved New France of supplies and helped cost France Canada in 1763. The tactic proved decisive again against Napoleon, and the Union blockade of southern ports shaped the American Civil War.
Admirals distinguish three styles. A close blockade keeps warships in sight of the port: the most effective approach, but exhausting, stormy and exposed to sudden sorties. A distant blockade intercepts ships far offshore, needing more vessels but carrying less risk. A loose blockade withdraws just beyond the horizon, which is how Nelson coaxed Villeneuve's Franco-Spanish fleet out of Cádiz. Success depends on the value of what is being stopped, superior strength, knowledge of the enemy's routes, and patience, since blockades tend to run for years.
The law has evolved too. The 1827 action to aid Greek rebels against Turkey, never backed by a declaration of war, is counted as the first pacific blockade, and Britain's 1837 blockade of New Granada to free a jailed consul involved no shooting at all. Under modern international law a blockade is an act of war, lawful only in self-defence; using one to starve or collectively punish civilians is a war crime.
Source: Blockade
