Invasion
In geopolitics, an invasion typically refers to a military offensive in which a polity sends combatants, usually in large numbers, to forcefully enter the territory of another polity, with either side possibly being supported by one or more allies. While strategic goals for an invasion can be numerous and complex in nature, the foremost tactical objective normally involves militarily occupying part or all of the invaded polity's territory. Today, if a polity conducts an invasion without having been attacked by their opponent beforehand, it is widely considered to constitute an international crime and condemned as an act of aggression.
Historically, invasions have variously been associated with conquest and annexation, self-defence, liberation of the invaded polity's people, or the establishment or re-establishment of control or authority over a territory. Other common motives include forcing the partition of a polity, toppling or altering the established government of a polity or gaining concessions from said government, or intervening to support a belligerent in a polity's civil war.
An invasion can be the cause of a war, be a part of a larger strategy to end a war, or it can constitute an entire war in itself. Due to the large scale of the military operations associated with invasions, they are usually strategic in planning and execution.
History
evidence indicates that invasions have been frequent occurrences since prehistory. In antiquity, before radio communications and fast transportation, the only way for a military to ensure adequate reinforcements was to move armies as one massive force. This, by its very nature, led to the strategy of invasion. With invasion came cultural exchanges in government, religion, philosophy, and technology that shaped the development of much of the ancient world.Defenses
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 a 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 Iraq 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 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.
Methods
There are many different methods by which an invasion can take place, each method having arguments both in their favour and against. These include invasion by land, sea, or air, or any combination of these methods.By land
Invasion over land is the straightforward entry of armed forces into an area using existing land connections, usually crossing borders or otherwise defined zones, such as a demilitarized zone, overwhelming defensive emplacements and structures. Although this tactic often results in a quick victory, troop movements are relatively slow and subject to disruption by terrain and weather. Furthermore, it is hard to conceal plans for this method of invasion, as most geopolitical entities take defensive positions in areas that are the most vulnerable to the methods mentioned above.In modern warfare, invasion by land often takes place after, or sometimes during, attacks on the target by other means. Air strikes and cruise missiles launched from ships at sea are a common method of "softening" the target. Other, more subtle, preparations may involve secretly garnering popular support, assassinating potentially threatening political or military figures, and closing off supply lines where they cross into neighboring countries. In some cases, those other means of attack eliminate the need for ground assault; the 1945 atomic-bombing of Hiroshima and Nagasaki ultimately made it unnecessary for the Allies to invade the Japanese home islands with infantry troops. In cases such as this, while some ground troops are still needed to occupy the conquered territory, they are allowed to enter under the terms of a treaty and as such are no longer invaders. As unmanned, long-range combat evolves, the instances of basic overland invasion become fewer; often the conventional fighting is effectively over before the infantry arrives in the role of peacekeepers.
By sea
Invasion by sea is the use of a body of water to facilitate the entry of armed forces into an area, often a landmass adjoining the body of water or an island. This is generally used either in conjunction with another method of invasion, and especially before the invention of flight, for cases in which there is no other method to enter the territory in question. Arguments in favor of this method usually consist of the ability to perform a surprise attack from sea, or that naval defenses of the area in question are inadequate to repel such an attack. However, the large amount of specialized equipment, such as amphibious vehicles and the difficulty of establishing defenses—usually with a resulting high casualty count—in exchange for a relatively small gain, are often used as arguments against such an invasion method. Underwater hazards and a lack of good cover are very common problems during invasions from the sea. At the Battle of Tarawa, Marine landing craft became hung up on a coral reef and were shelled from the beach. Other landers were sunk before they could reach the shore, and the tanks they were carrying were stranded in the water. Most of the few survivors of the first wave ended up pinned down on the beach. The island was conquered but at a heavy cost, and the loss of life sparked mass protests from civilians in the United States.By air
Invasion by air is an invention of the 20th century and modern warfare. The idea involves sending military units into a territory by aircraft. The aircraft either land, allowing the military units to debark and attempt their objective, or the troops exit the aircraft while still in the air, using parachutes or similar devices to land in the territory being invaded. Many times air assaults have been used to pave the way for a ground- or sea-based invasion, by taking key positions deep behind enemy lines such as bridges and crossroads, but an entirely air-based invasion has never succeeded. Two immediate problems are resupply and reinforcement. A large airborne force cannot be adequately supplied without meeting up with ground forces; an airborne force too small simply places themselves into an immediate envelopment situation. Arguments in favor of this method generally relate to the ability to target specific areas that may not necessarily be easily accessible by land or sea, a greater chance of surprising the enemy and overwhelming defensive structures, and, in many cases, the need for a reduced number of forces due to the element of surprise. Arguments against this method typically involve capacity to perform such an invasion—such as the sheer number of planes that would be needed to carry a sufficient number of troops—and the need for a high level of intelligence in order for the invasion to be successful.The closest examples to a true air invasion are the Battle of Crete, Operation Thursday and Operation Market Garden. The latter was an assault on the German-occupied Netherlands conducted in September 1944. Nearly 35,000 men were dropped by parachute and glider into enemy territory in an attempt to capture bridges from the Germans and make way for the Allies' advance. However, even with such a massive force taking the Germans completely by surprise, the assault was a tactical failure and after 9 days of fighting the Allies managed only to escape back to their own lines, having sustained over 18,000 casualties. In the 21st century, as vast improvements are made in anti-aircraft defenses, it seems that the air invasion is a strategy whose time may never come.