In Civilization IV: Beyond the Sword, trade routes across water tiles can now be blockaded by enemy ships. The "Blockade Mission" can only be performed by Naval Units and is most effective when performed in close proximity to an enemy city. In general, this means that the "Blockade" mission works best between players at war. The lone exception to this rule is when players use the Privateer unit, which can effectively blockade at any time.
By choosing the "Blockade Mission," a Naval Unit may prevent enemy trade routes from passing through the area which they are blockading, effectively preventing enemies from gaining any gold from that route. Blockaded regions appear as large striped zones stretching outward from the unit performing the mission. If an enemy city falls within this zone, players can ensure that no overseas trade will reach this city! If a city is blockaded by a Privateer while two nations are at peace, the blockading player will receive a portion of the gold from the blockaded routes.