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In geopolitics, an invasion typically refers to a military offensive in which a polity sends combatants, usually in large numbers, to forcefully enter the territory of another polity,^[1] with either side possibly being supported by one or more allies. While strategic goals for an invasion can be numerous and complex in nature, the foremost tactical objective normally involves militarily occupying part or all of the invaded polity's territory. Today, if a polity conducts an invasion without having been attacked by their opponent beforehand, it is widely considered to constitute an international crime and condemned as an act of aggression.

Historically, invasions have variously been associated with conquest and annexation, self-defence (if the invader was attacked first), liberation of the invaded polity's people (or of territory that had been occupied by it), or the establishment or re-establishment of control or authority over a territory. Other common motives include forcing the partition of a polity, toppling or altering the established government of a polity or gaining concessions from said government, or intervening to support a belligerent in a polity's civil war.^{[citation needed]}

An invasion can be the cause of a war, be a part of a larger strategy to end a war, or it can constitute an entire war in itself. Due to the large scale of the military operations associated with invasions, they are usually strategic in planning and execution.^{[not verified in body]}