An invasion is a military offensive consisting of all, or large parts of the armed forces of one geopolitical entity aggressively entering territory controlled by another such entity, generally with the objective of either conquering, liberating or re-establishing control or authority over a territory, forcing the partition of a country, altering the established government or gaining concessions from said government, or a combination thereof. An invasion can be the cause of a war, be used as 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 operations associated with invasions, they are usually strategic in planning and execution.

Overview

The term usually denotes a strategic endeavor of substantial magnitude; because the goals of an invasion are usually large-scale and long-term, a sizeable force is needed to hold territory, and protect the interests of the invading entity. Smaller-scale, tactical cross-border actions, such as skirmishes, sorties, raids, infiltrations or guerrilla warfare, are not generally considered invasions. A military endeavor to take back territory that is tenuously held by an initial invader during the course of war is generally called a counter-offensive rather than an invasion.

Military operations that occur within the territory of a single geopolitical entity can sometimes be termed an invasion if armed forces enter into a well defined part of that territory that, at the time of the operation, was completely under the control of armed forces of the other faction in a civil war or insurrection situation. For example, during both the American Revolutionary War and the American Civil War many of the military operations conducted during these wars are called invasions for this reason even though they did not involve "foreign" armies entering from "foreign" nations strictly speaking.

It should be noted that the term invasion does not, in and of itself, imply either a justified or unjustified course of action. For example, during World War II, German military operations conducted against Poland in 1939 are often called the Invasion of Poland while military operations conducted against Nazi controlled France in 1944 is called the Invasion of Normandy. Both military operations are properly called invasions because they involved an outside force entering territory not under its authority or control at the time. The morality or immorality of the military operation itself is not a factor in determining whether it is termed as an invasion.

History

Defenses

States with potentially hostile neighbors typically adopt defensive measures to delay or forestall an invasion. In addition to utilizing geographical barriers such as rivers, marshes, or rugged terrain, these measures have historically included fortifications. Such a defense can be intended to actively prevent invading forces from entering the country by means of an extended and well-defended barrier; the Great Wall of China, Hadrian's Wall, and the Danewerk are famous examples. Such barriers have also included trench lines and, in more modern times, minefields, cameras, and motion-sensitive sensors. However, these barriers can require a large military force to provide the defense, as well as maintain the equipment and positions, which can impose a great economic burden on the country. Some of those same techniques can also be turned against defenders, used to keep them from escape or resupply. During Operation Starvation, Allied forces used airdropped mines to severely disrupt Japanese logistical operations within their own borders.