A blockade is an effort usually (but not always, see below) at sea, to prevent supplies from reaching the enemy.

A blockade is also the attritional aspect of a siege, with the besiegers preventing food supplies from reaching the besieged. If the siege lasted long enough, defenders and civilians were reduced to eating anything vaguely edible to avoid starvation—family pets, the leather from shoes, and even each other. In modern times, the related "low-impact" attritional weapon of economic sanctions has been applied to blockade whole countries (for instance, Cuba, Libya and Iraq).

Historical blockades include:

• Union blockade of the Confederacy during the American Civil War
• British blockade of Germany during World War I as a part of the First Battle of the Atlantic.
• The Second Battle of the Atlantic during World War II
• United States blockade of Japan during World War II
• United States blockade of Cuba during the Cuban missile crisis in 1962
• Soviet land blockade of West Berlin, 1948–1949, known as the Berlin Blockade.