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 (if the invader was attacked first), liberation of the invaded polity's people (or of territory that had been occupied by it), 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.^{[citation needed]}

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.^{[not verified in body]}

Archaeological 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.

Before the days of package tours and cut-price airlines, military invasions functioned as a standard major form of proto-tourism – bringing large numbers of foreign visitors into new environments, with the consequential social, cultural and economic impacts on indigenous populations and on the invaders.

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.^{[citation needed]}