A blockade is an effort to cut off food, supplies, war material or communications from a particular area by force, either in part or totally. A blockade should not be confused with an embargo or sanctions, which are legal barriers to trade, and is distinct from a siege in that a blockade is usually directed at an entire country or region, rather than a fortress or city. While most blockades historically took place at sea, blockade is still used on land to prevent someone coming into a certain area. With the blockading power can seek to cut off all maritime transport from and to the blockaded country; although stopping all land transport to and from an area may also be considered a blockade. In the 20th century air power has also been used to enhance the effectiveness of the blockade by halting all air traffic within the blockaded air space.Close patrol of the hostile ports, in order to prevent naval forces from putting to sea, is also referred to as a blockade. When coastal cities or fortresses were besieged from the landward side, the besiegers would often blockade the seaward side as well. Most recently, blockades have sometimes included cutting off electronic communications by jamming radio signals and severing undersea cables.