A military invasion is a large-scale military operation in which the armed forces of one geopolitical entity aggressively enters territory controlled by another such entity. An invasion can occur within the framework of a war or it may be an independent conflict. Invasions typically result in occupation of the territory, and in many cases profoundly change the culture of the inhabitants, as well as the political landscape.
The motivation for an invasion varies widely. It can range from a desire for immediate looting of the conquered territory, to an interest in gaining control over a country and its resources, to the acquisition of natural resources such as water and petroleum supplies; the pursuit of enemies; religious idealism; policies of national interest; preemption of a real or perceived future attack; or quelling destabilizing, unconscionable conflict among or between neighbors.
Because an invasion is usually a very large operation, it requires a very large number of troops and supplies. It’s important that the advancing force outnumbers the defenders by a significant amount. This is why many operations that involve airborne deployments of soldiers or the use of naval and amphibious forces are referred to as invasions, although they do not actually involve an ‘invasion’ in the strict sense of the word.
Depending on the circumstances, invaders can attempt to mitigate the impact of an invasion by creating a series of spaced fortifications along the border between themselves and the defending nation. These structures can delay an invasion until the defender has time to organize a defense, and can also interdict the invasion’s supply lines, as the Maginot Line did for the Allies on D-Day in June of 1944.