Military invasion is the aggressive entry of armed forces of one geopolitical entity into territory controlled by another such entity, generally for the purpose of occupying or conquering that territory. It may be motivated by a desire to reclaim or assert control over previously lost territory, retaliation for real or perceived wrongdoing, seizing natural resources or strategic positions, effecting a change in government, or some combination of the above. Invasion is usually large-scale, and smaller operations are classified as skirmishes, sorties, targeted killings or reconnaissance in force. Invasion by air is a modern invention, but it is commonly used in conjunction with an invasion by land or sea, or as a way to “soften up” the target before launching a ground assault. This preparation can take the form of a variety of actions including secretly garnering popular support, assassinating potentially threatening political and military figures, closing off supply lines (such as bridges) near the border and/or bombarding the target with air strikes and cruise missiles launched from ships at sea.
Invasion is typically a long-term operation, and there are many logistical and other challenges to be overcome. For example, a lack of sufficient transports to move the entire army in advance of an attack can be catastrophic. Likewise, invading forces often find themselves isolated from their homeland, cut off from their normal supply sources and facing the prospect of being annihilated by starvation, cold weather or other factors. This is why it’s important for invaders to have a substantial merchant navy, and why their officers must keep word of an impending attack from leaking out to the enemy.