Invasion

The term invasion 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 instead generally called a counter-offensive.

The term does not imply the presence or lack of justification for the action, and the morality or immorality of a military operation does not determine whether it is so termed. For example, two sets of World War II military operations—by Germans against Poland in 1939 and by Allies against Nazi controlled France in 1944—are often called the Invasion of Poland and Invasion of Normandy, respectively. Both military operations are properly called invasions because they involved an outside force entering territory not under its authority or control at the time.

Archaeological evidence indicates that invasions have been frequent occurrences since prehistory. In antiquity, before radio communications and fast transportation, the only way to ensure adequate reinforcements was to move armies as one massive force. This, by its very nature, led to the strategy of invasion. With invasion came cultural exchanges in government, religion, philosophy, and technology that shaped the development of much of the ancient world.

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.

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