Constructing defences at land, water and air frontiers to prevent entry of opposing military forces.
Fortified frontier lines appeared as walls in China, the Middle East and Egypt to prevent raiders and neighbouring armies from entering home territory. In Europe this developed into lines of watch towers, and fortresses along important rivers or trade routes. As naval warfare increased, coastal waters, sea lanes and rivers were protected by batteries, mines and ships. Fortified airspace and even outer atmosphere defence became important during the 20th century.
Fortifications provide deterrence to foreign enemies. Peaceful stability can be maintained by strong border defences.
Increasingly mobile military forces can avoid stationary fortresses (i.e. the Maginot Line in World War II).