Invasion is a military action in which the armed forces of one geopolitical entity enter another territory with the intent of conquest or occupation. A military invasion may be part of a war or an independent conflict, and it usually profoundly changes the culture and political landscape of the invaded country. A modern political trend, probably motivated by a desire to avoid charges of imperialism, has been for the invader to euphemistically label the operation as an intervention that will achieve a goal stated in beneficial terms.
Invasions can be conducted by land, sea or air. Each method has advantages and disadvantages. A ground invasion involves moving a large military force over long distances. This is time consuming and costly. It is also susceptible to disruption due to weather and terrain. An air-based invasion is more rapid, but requires a great deal of advanced technology to ensure that military units are dropped on target and do not get lost in hostile territory. It is difficult to conceal plans for an air-based invasion and is also vulnerable to counterattack.
Invasions can be mitigated by constructing spaced fortifications along a border with the defending nation. These can delay an attack long enough to allow the defender to mobilize a larger army for defense. In addition, they can interdict the invader’s supply lines, as the Maginot Line did for Allied troops on D-Day in 1944. However, this tactic can also draw a large invasion too deep into contested territory and leave it exposed to a pincer movement.