A military invasion is an action in which a sizable number of combatants of one geopolitical entity aggressively enter the territory of another, usually with the intent to conquer the area and change its government. This action normally constitutes an aspect of a war, but can also be the cause of such a war. Justifications for such an action often include restoration of territory lost in the past; religious idealism; pursuit of enemies; policies of national interest; retaliation against perceived wrongdoing, protection or acquisition of transportation routes or natural resources, including water and petroleum; quelling destabilizing or unconscionable conflict within or between neighbors; and other reasons.

Historically, invasions have been most commonly mounted by land or sea. Invasions by sea can be accomplished using amphibious vehicles or with a naval presence. Invasions by land typically require a large amount of specialized equipment and can be very difficult to implement, especially before the invention of flight.

Attempts to mitigate the impact of an invasion can include the use of fortifications. These may be designed to delay an attack until the defending nation can organize its defense, as in the case of Hadrian’s Wall and the Great Wall of China or the Danewerk, or interdict the invader’s supply lines, as in the Maginot Line. Many nations with potentially hostile neighbors adopt defensive measures to delay or forestall an invasion; such barriers can be costly in terms of the specialized equipment needed, and the maintenance requirements to keep these defended positions operational.