A blockade is when one group uses ships, soldiers, or other forces to prevent people or supplies from entering or leaving a place. During the American Civil War, the Union Navy blockaded Southern ports to stop the Confederacy from shipping cotton to Europe or receiving weapons and supplies in return. Ships patrolled the coastline, turning away or capturing any vessels that tried to break through.

Blockades have shaped major moments in history. During World War II, German submarines tried to blockade Britain by sinking supply ships crossing the Atlantic Ocean. The Berlin Blockade of 1948 saw the Soviet Union close all roads and railways into West Berlin, hoping to force the Allies out, but American and British planes flew in food and fuel for nearly a year until the Soviets gave up.

The word can also describe non-military situations. Police might blockade a street during a parade to keep cars out. Protesters sometimes create a blockade by linking arms or parking vehicles across a road.

A successful blockade requires patience and resources: you need enough ships, troops, or barriers to actually seal off access, and you need to maintain that seal long enough to matter. Breaking through a blockade, called running the blockade, is dangerous but sometimes necessary when people inside desperately need supplies.