What is Invasion?

Legal Definition
An invasion is a military offensive in which large parts of combatants of one geopolitical entity aggressively enter 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 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.
-- Wikipedia
Legal Definition
The entry of a country by a public enemy, making war.

2. The Constitution of the United States, art. 1, s. 8, gives power to congress "to provide for calling the militia to execute the laws of the Union, suppress insurrections, and repel invasions." Vide Insurrection.
-- Bouviers Law Dictionary
Legal Definition
An encroachment upon the rights of another; the incursion of an army for conquest or plunder. Webster. See Act-na Ins. Co. v. Boon, 95 U. S. 129, 24 L. Ed. 395.
-- Black's Law Dictionary
Legal Definition
A hostile entry into a country by a public enemy.
-- Ballentine's Law Dictionary