Invasion
Invasion is a military action consisting of troops entering a foreign land (a nation or territory, or part of that), often resulting in the invading power occupying the area, whether briefly or for a long period. Euphemistically, an invasion is sometimes referred to as an intervention.
Invasions may be conducted to change or restore the leadership of a nation of territory. This sort of invasion is likely to be perceived by one side as an act of usurpation and by the other as an act of liberation. Invasion is often opposed by native resistance.
Other historical reasons for invasion have included restoration of territory lost in the past, expansion of territory, pursuit of enemies, protection of allies, pre-emption of an attack, protection or acquisition of transportation routes and water or oil supplies, quelling destabilizing or unconscionable conflict within or between neighbours, and punishment for a perceived slight.