Military invasion is the use of armed force by a country or a consortium of allied countries to enter territory that was not under their control or ownership at the time of the attack. The goal of this type of military operation is usually to conquer, liberate, or re-establish control over the contested territory. This type of large-scale military action may be referred to as an invasion when it is done by land, air, or sea, but many more minor actions such as skirmishes, sorties, targeted killings and assassinations, or even reconnaissance infiltrations are not considered to be an invasion by the international community.
Invasions may also be used to occupy a country, which requires a much more substantial effort than simply a ground assault. This often involves a slow build-up of forces that eventually culminate in a military victory. This may involve reducing membership in covert groups that are threatening to the occupying force, interdicting supply lines of these groups, or reeducating the people so prolonged resistance is unlikely.
Invasions that take place at sea are normally conducted after, or during, attacks on the target country by other means. These other means can include air strikes, laying mines at sea or in waterways, or the use of cruise missiles from ships in the area. Underwater hazards are also a common problem for invaders who attack by sea. For example, the Marine landings at Tarawa during World War II were hampered by underwater ridges and coral reefs that resulted in a high number of casualties.