Military invasion is the military action in which a sizable number of combatants of one geopolitical entity aggressively enter another polity’s territory. The intent of an invasion can be conquest of territory, alteration of the established government, or a combination thereof. Military invasion is often part of a war, but may be conducted independent of one.
Invasion can have a significant impact on the culture of the occupied territory and its inhabitants. The invasion can also change the political landscape of the region and alter international relations. The motives for an invasion vary greatly, and have included the desire to loot conquered territories, gain access to natural resources and strategic positions; retaliation for real or perceived attacks by an enemy, effecting a change in the ruling government, or a combination of these reasons.
During times of possible threat, nation-states with hostile neighbors have traditionally adopted defensive measures to delay or forestall an invasion. This has included building a series of spaced fortifications along a border. The structures can delay an invasion until the defender has sufficient forces to organize a defense, and can interdict the invader’s supply lines by blocking their routes to reinforcements—as the Maginot Line did for the Allies on D-Day in 1944.
Another form of defensive action is to create a fortress within an urban area, using existing buildings that are reinforced with lumber and steel girders to make them hard to destroy, creating bunkers in the process. This type of fortification is expensive and requires a large force to protect, and can still be compromised by modern technology such as minefields and camera systems.
