States with potentially hostile neighbors typically adopt defensive measures to delay or forestall an invasion. In addition to utilizing geographical barriers such as rivers, marshes, or rugged terrain, these measures have historically included fortifications. Such a defense can be intended to actively prevent invading forces from entering the country by means of an extended and well-defended barrier; the Great Wall of China, Hadrian's Wall, and the Danewerk are famous examples. Such barriers have also included trench lines and, in more modern times, minefields, cameras, and motion-sensitive sensors. However, these barriers can require a large military force to provide the defense, as well as maintain the equipment and positions, which can impose a great economic burden on the country. Some of those same techniques can also be turned against defenders, used to keep them from escape or resupply. During Operation Starvation, Allied forces used airdropped mines to severely disrupt Japanese logistical operations within their own borders.
Alternatively, the fortifications can be built up at a series of sites, such as castles or forts placed near a border. These structures are designed to delay an invasion long enough for the defending nation to mobilize an army of size sufficient for defense or, in some cases, counter-invasion—such as, for example, the Maginot Line. Forts can be positioned so that the garrisons can interdict the supply lines of the invaders. The theory behind these spaced forts is that the invader cannot afford to bypass these defenses, and so must lay siege to the structures.
In modern times, the notion of constructing large-scale static defenses to combat land-based threats has largely become obsolete. The use of precision air campaigns and large-scale mechanization have made lighter, more mobile defenses desirable to military planners. The obsolescence of large fortifications was displayed by the failure of the Maginot Line in the beginning of World War Two. Nations defending against modern invasions normally use large population centers such as cities or towns as defensive points. The invader must capture these points to destroy the defender's ability to wage war. The defender uses mobile armored and infantry divisions to protect these points, but the defenders are still very mobile and can normally retreat. A prominent example of the use of cities as fortifications can be seen in the Iraqi Army's stands in the 2003 invasion of Iraq at Baghdad, Tikrit and Basra in the major combat in the Second Gulf War. A defender can also use these mobile assets to precipitate a counteroffensive like the Soviet Red Army at the Battle of Kursk or the Northern Alliance in Afghanistan.
However, static emplacements remain useful in both defense against naval attacks and defense against air attacks. Naval mines are still an inexpensive but effective way to defend ports and choke off supply lines. Large static air defense systems that combine antiaircraft guns with missile launchers are still the best way to defend against air attacks. Such systems were used effectively by the North Vietnamese around Hanoi. Also, the United States has invested considerable time and money into the construction of a National Missile Defense system, a static defense grid intended to intercept nuclear intercontinental ballistic missiles.
Island nations, such as the United Kingdom or Japan, and continental states with extensive coasts, such as the United States, have utilized a significant naval presence to forestall an invasion of their country, rather than fortifying their border areas. A successful naval defense, however, usually requires a preponderance of naval power and the ability to sustain and service that defense force.
In particularly large nations, the defending force may also retreat in order to facilitate a counterattack by drawing the invaders deeper into hostile territory. One effect of this tactic is that the invading force becomes too spread out, making supply difficult and making the lines more susceptible to attack. This tactic, although costly, helped the Soviets stop the German advance at Stalingrad. It can also cause the invading force to extend too far, allowing a pincer movement to cut them off from reinforcements. This was the cause of the British defeat at the Battle of Cowpens during the American Revolutionary War. Finally, sending too many reinforcements can leave too few defenders in the attackers' territory, allowing a counter-invasion from other areas, as happened in the Second Punic War.
Read more about this topic: Invasion
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“We are a nation of politicians, concerned about the outmost defenses only of freedom. It is our childrens children who may perchance be really free.”
—Henry David Thoreau (18171862)