Military invasion is the act of a sizable number of armed forces from one geopolitical entity aggressively entering territory controlled by another such entity, with the aim of conquering, recapturing or liberating it, retaliating for real or perceived wrongdoings, forcing partition, altering the established government, gaining concessions to natural resources or strategic positions or any combination thereof. An invasion can be the cause of a war, a war’s main objective or an independent war in itself.
Invasions are often large-scale military operations requiring a significant number of soldiers and considerable planning. Due to the nature of their scope, the consequences of an invasion tend to go beyond determining who will control the land for an extended period of time. On many occasions the societal development of the invaded territories has been significantly altered as a result of interactions among the invading and the invaded cultures.
For example, the Allied invasion of Germany in WWII was carried out by air, sea and ground forces. A defining feature of the landing on the beaches was that they were heavily protected by minefields and anti-tank guns, and the military units could only fight delaying actions at choke points and other defensible positions until they ran out of ammunition or were overrun by the enemy.
Invasion by sea is a relatively common method for invasion, especially before the invention of flight, as it allows for a surprise attack and the ability to enter a country over a body of water or an island. However, it requires a great deal of specialized equipment and is extremely risky. For instance, Marine landing craft were sunk at the Battle of Tarawa or hung up on coral reefs and were subjected to heavy shelling from inland.