Military invasion is a military action in which armed forces of one geopolitical entity cross borders into another territory with the intent of conquest, occupation or the establishment of control. Generally speaking, invasions involve the large-scale deployment of a substantial military force to the point that it becomes a significant presence in a region and that it can be identified by the local populace as foreign.
Defending an area against invasion requires considerable resources; the more territory a defense is expected to cover, the larger the military force and the resources required to control it. The cost of these resources can be quite high, especially for a defender if they are caught off guard by an invader that has the ability to strike at will from multiple directions.
Invasions can be accomplished by land, sea or air; the latter is particularly challenging when the terrain is replete with hazards, such as forests, swamps and mountains. It is often very difficult for a militarized force to gain and hold ground in such conditions, which may lead to significant casualties and delay the progress of the invasion.
The success of a military invasion depends on the invading force’s ability to establish good will with the people of the invaded country and create new habits that reflect respect for human rights. The more repressive the previous institutions were, the more difficult it will be for an occupying force to achieve this goal.