Military invasion is the process in which a sizable military force of one geopolitical entity aggressively enters territory controlled by another such entity. It may be within the context of a war or it may stand alone. The result is often occupation of the territory and, for the occupying power, profound changes in its culture and political landscape. It may also cause the dissolution of its government, the granting of client states or the acquisition of natural resources and strategic positions.
Invasion has been a regular feature in the history of warfare, dating back thousands of years. The motivations have included restoration of territory lost in the past; religious idealism; national interest policy; pursuit of enemies or to preempt a potential attack; acquisition of colonies; retaliation for real or perceived actions by neighbors; gaining access to natural resources, such as water and petroleum; forcing the partition of a country; effecting a change in its ruling government; or any combination of these and other reasons.
Defending against a military invasion requires substantial resources, and the larger the area to defend, the more expensive those resources must be. This can make a military invasion costly and dangerous for the invader, which may result in a loss of troops or other resources.
Nations defending against an invasion usually employ defensive measures that include the construction of fortifications. These can be designed to block the invader by preventing it from entering, such as Hadrian’s Wall or the Great Wall of China, or they can be spaced along a line of the border, with garrisons interdicting the invader’s supply lines.
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