Invasion

An invasion is a military offensive where one polity sends combatants to forcefully enter the territory of another.

The primary tactical objective is usually the military occupation of all or part of the target territory. While goals can be complex, they often involve conquest or annexation. Today, if a polity conducts an invasion without being attacked first, it is widely considered an international crime. This act is officially condemned as an act of aggression. Invasions can be the cause of a war or a single part of a larger strategy. They can even constitute an entire war by themselves.

Historically, invasions have occurred since prehistory. In antiquity, the lack of radio and fast transport shaped how armies moved. To ensure they had enough reinforcements, militaries moved as one massive force. This necessity led to the specific strategy of invasion. These movements often caused cultural exchanges in religion, philosophy, and technology. Such exchanges helped shape the development of the ancient world.

There are three primary methods for conducting an invasion: land, sea, and air. Land invasions involve moving armed forces across existing land connections or borders. This method can result in quick victories but is often slow due to weather or terrain. Modern land invasions might begin with air strikes to "soften" the target first. Sea invasions use bodies of water to reach landmasses or islands. This can allow for surprise attacks if naval defenses are weak. However, sea invasions often involve high casualty counts and specialized equipment like amphibious vehicles.

Air invasion is a more recent development from the 20th century. This method involves using aircraft to land troops or drop them via parachutes. Air assaults are often used to seize key positions like bridges behind enemy lines. While they offer surprise, an entirely air-based invasion has never succeeded on its own. Airborne forces face immediate problems with resupply and reinforcement. They must eventually meet up with ground forces to remain effective.

States use various methods to defend against these threats. Some rely on geographical barriers like rivers or rugged terrain. Others build large fortifications to prevent entry. The Great Wall of China and Hadrian's Wall are famous examples of these barriers.

Some nations build a series of spaced forts or castles near borders. These are designed to delay an invader and force them to lay siege to specific sites. The Maginot Line was a notable series of such fortifications. However, the failure of the Maginot Line in World War Two showed that large static defenses can become obsolete.

Modern military planning has shifted toward more mobile defenses. Large-scale static fortifications are often replaced by the use of cities as defensive points. An invader must capture these population centers to destroy a defender's ability to wage war. Defenders use mobile armored and infantry divisions to protect these areas. For example, the Iraqi Army used cities like Baghdad and Basra during the 2003 invasion of Iraq.

Some nations, like the United Kingdom or Japan, rely on a strong naval presence instead of border fortifications.

Defensive tactics can also involve strategic retreats. In large nations, a defender might draw an invader deeper into hostile territory. This can cause the invading force to become too spread out. When supply lines are stretched, they become susceptible to attack. The Soviet Union used this tactic to stop the German advance at Stalingrad. However, such tactics can also lead to a pincer movement that cuts off reinforcements. This occurred during the Battle of Cowpens in the American Revolutionary War.