Military invasion is a military offensive in which a sizable number of combatants of one geopolitical entity enter the territory of another such entity with the intention of conquering, re-establishing control, retaliation for real or perceived actions, liberation of previously lost territory, forcing the partition of a country, gaining concessions or access to natural resources or strategic positions, effecting a change in the ruling government, or any combination of these objectives. Regardless of the reasons behind an invasion, a successful outcome typically results in either the transformation of a nation into a client state with requirements to pay reparations or tribute to the victorious polity or a return to the status quo prior to the war.
Invasions that occur at sea typically require large amounts of specialized equipment, and the defenders can normally outgun any invaders. The difficulty of establishing defenses, in addition to the high casualty count, make such operations controversial.
The success of an invasion often depends on the ability to win popular support, especially after the regular army is defeated and a new government established. Invasion also requires the occupying force to establish good will with the inhabitants of the region and create habits that reflect respect for human rights. Failure to accomplish this may result in continued opposition from covert groups, which are difficult to defeat in the long term. Invasions in modern times are normally fought through the use of drones, airborne units, and land-based assault vehicles.