An invasion is a large-scale military operation in which the armed forces of one geopolitical entity aggressively enter territory controlled by another. It may be done within the framework of a war or it may be an independent conflict. The objective is often conquering or liberating the territory, changing its established government, or a combination of both.
Invasions can be carried out by land, sea, air, or a combination of these, with each method having advantages and disadvantages. The most straightforward way to invade is by land, which involves crossing borders or otherwise defined zones and overwhelming defensive emplacements and structures. However, this requires extensive troop movements that can be slowed down by weather and terrain. It can also expose invaders to pincer attacks, with enemy forces in a position to cut off resupply and reinforcement.
A naval invasion is an alternative, involving ships and submarines. These can travel under water to bypass defended shorelines, but they face the same problems as land-based invasions in terms of speed and terrain. An air-based invasion is more flexible, but it requires aircraft that can carry sufficient troops to meet the target’s needs. In addition, an entirely air-based invasion has never succeeded.
Regardless of the method chosen, it is important to pacify the occupied territory once the regular military has been defeated. Various methods can be used for this, including propaganda, reeducation, or simply providing food, shelter and other necessities. In extreme cases, the death or imprisonment of a prominent leader can be enough to discourage further resistance.