- Date: 15th century
- 1 : an act of invading; especially : incursion of an army for conquest or plunder
- 2 : the incoming or spread of something usually hurtful
- 3 : The spreading of pathogenic microorganisms or malignant cells that are already in the body to new sites.
- 4 : The spread of a plant or animal population into an area formerly free of the species concerned.
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, 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.
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.
Military operations that occur within the territory of a single geopolitical entity are sometimes termed "invasions" 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 another faction in a civil war or insurrection. For example, many such operations of both the American Revolutionary War and the American Civil War are called invasions even though they did not involve entry of armies from foreign nations.
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.