Great walls and border forts could only delay an invasion
Hadrian's Wall, the Great Wall of China and the Danish Danewerk all tried to stop invaders at the frontier. Such barriers demanded huge garrisons and constant upkeep. Chains of forts had a humbler goal: hold out long enough for the defender to raise an army, since an enemy could not safely leave them standing behind it.
Invasions, in which one polity sends large forces to seize another's territory, appear in the archaeological record from prehistoric times. In the ancient world, without radio or fast transport, the only way to keep an army reinforced was to move it as one great mass, which made invasion the natural strategy. Those movements also spread ideas about government, religion, philosophy and technology across much of the ancient world. Today an unprovoked invasion is widely regarded as an international crime of aggression.
Defenders have long exploited rivers, marshes and mountains alongside man-made works. Continuous barriers sought to keep attackers out entirely; spaced castles and forts instead forced sieges and threatened invaders' supply lines. The Maginot Line in France followed this logic and failed at the start of the Second World War, a sign that massive static defences had been outpaced by mechanised armies and air power. Static systems still earn their keep against ships and aircraft, however: naval mines remain a cheap way to guard ports.
Island nations such as Britain and Japan, and coastal giants such as the United States, have relied chiefly on naval strength instead of border fortresses, although that demands a dominant, well-supplied fleet.
Large countries can also trade space for time, falling back to lure invaders deep inside, where their supply lines stretch thin and grow exposed. That costly approach helped the Soviets halt the German advance at Stalingrad. An overextended attacker may also be caught in a pincer, which is how the British lost the Battle of Cowpens in the American Revolutionary War. And pouring troops into a campaign abroad can strip the homeland bare, inviting a counter-invasion, as happened in the Second Punic War. Land invasions are the most direct method, but they move slowly, suffer from terrain and weather, and are hard to keep secret.
Source: Invasion
