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An invasion is a military action consisting of armed forces of one geopolitical entity entering territory controlled by another such entity, generally with the objective of conquering territory or altering the established government. An invasion can be the cause of a war, it can be used as a part of a larger strategy to end a war, or it can constitute an entire war in and of itself.

The term usually connotes a strategic endeavor of substantial magnitude; because the goals of an invasion are usually large-scale and long-term, large forces are needed to hold territory and protect the interests of the invading entity. Smaller and lighter tactical infiltrations are not generally considered invasions, being more often classified as skirmishes, sorties, targeted killings, assassinations or reconnaissance in force. By definition, an invasion is an attack from outside forces. As such, rebellions, civil wars, coups d'etat, and internal acts of democide or other acts of oppression are generally not considered invasions.

Invasion as aggression

After World War II, the Nuremberg Tribunal was convened to prosecute Nazi Germany officials for war crimes. In that context, the concept of "invasion" was abstracted in principle as "aggression," to refer to such hostility itself as a violation of principles prohibiting war crimes. Germany had begun a campaign of offenses that included the invasions of almost all neighboring countries in Europe, including Poland, France, and Russia. According to the Tribunal, aggression is:

"the supreme international crime differing only from other war crimes in that it contains within itself the accumulated evil of the whole."

This principle is referred to in the Nuremberg Principles, which, in parallel with the Geneva Conventions, is included within the United Nations Charter.


Archaeological evidence indicates that invasions have been frequent 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 the cultural exchanges in government, religion, philosophy, and technology that shaped the development of much of the ancient world.

Motives and justifications

Invasions have normally been mounted for straightforward territorial gain and geopolitical advantage. Added to these motivations have often been the attractions of immediate looting. However, the reasons advanced as justifications of invasions have included restoration of territory lost in the past; religious idealism; policies of national interest; pursuit of enemies; protection of allies; acquisition of colonies; preemption of a real or perceived future attack; protection or acquisition of transportation routes or natural resources, including water and petroleum supplies; quelling destabilizing or unconscionable conflict within or between neighbors; and as punishment for a perceived slight.

A relatively recent justification for invasion, which arose during the nineteenth century with Great Powers assuming the right to arrange world politics, has been to change or restore the leadership or political regime of a nation or territory. Often in these cases, opposing powers cited the motive of "protection" of the invaded territory. During the nineteenth century, invasions of this nature frequently manifested themselves under the banner of imperialism. Such invasions are likely to be perceived by one side as an act of usurpation and by the other as an act of liberation.

A modern political trend, probably instigated by a desire to avoid charges of imperialism, has been for the invader to euphemistically label an invasion as an "intervention" to achieve a goal that is stated in beneficial terms.


The Great Wall in winter, near Beijing.
The Great Wall in winter, near Beijing.

Nation-states with potentially hostile neighbors will 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; Hadrian's Wall and the Great Wall of China 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.

Alternately, the fortifications can be built up at a series of sites, such as castles or forts placed near a border. These structures are designed to delay an invasion long enough for the defending nation to mobilize an army of size sufficient for defense or, in some cases, counter-invasion, such as for example, the Maginot Line. Forts can be positioned so that the garrisons can interdict the supply lines of the invaders. The theory behind these spaced forts is that the invader cannot afford to bypass these defenses, and so must lay siege to the structures.

The view from a battery at Ouvrage Schoenenbourg in Alsace. Notice the retractable turret in the right foreground.
The view from a battery at Ouvrage Schoenenbourg in Alsace. Notice the retractable turret in the right foreground.

In modern times, the notion of constructing large-scale static defenses to combat land based threats has largely become obsolete. The use of precision air campaigns and large-scale mechanization have made lighter, more mobile defenses desirable to military planners. Nations defending against modern invasions normally use large population centers such as cities or towns as defensive points. The invader must capture these points to destroy the defender's ability to wage war. The defender uses mobile armored and infantry divisions to protect these points, but the defenders are still very mobile and can normally retreat. A prominent example of the use of cities as fortifications can be seen in the Iraqi Army's stands in the 2003 Invasion of Iraq at Baghdad, Tikrit and Basra in the major combat in the Second Gulf War. A defender can also use these mobile assets to precipitate a counteroffensive like the Soviet Red Army at the Battle of Kursk or the Northern Alliance in Afghanistan.

However, static emplacements remain useful in both defense against naval attacks and defense against air attacks. Naval mines are still an inexpensive but effective way to defend ports and choke off supply lines. Large static air defense systems that combine antiaircraft guns with missile launchers are still the best way to defend against air attacks. Such systems were used effectively by the North Vietnamese around Hanoi. Also, the United States has invested considerable time and money into the construction of a National Missile Defense system, a static defense grid capable of intercepting nuclear intercontinental ballistic missiles.

Island nations, such as the United Kingdom or Japan, and continental states with extensive coasts, such as the United States, have utilized a significant naval presence to forestall an invasion of their country, rather than fortifying their border areas. A successful naval defense, however, usually requires a preponderance of naval power and the ability to sustain and service that defence force.

In particularly large nations, the defending force may also retreat in order to facilitate a counter attack by drawing the invaders deeper into hostile territory. One effect of this tactic is that the invading force becomes too spread out, making supply difficult and making the lines more susceptible to attack. This tactic, although costly, helped the Russians stop the German advance at Stalingrad. It can also cause the invading force to extend too far, allowing a pincer movement to cut them off from reinforcements. This was the cause of the British defeat at the Battle of Cowpens during the American Revolutionary War. Finally, sending too many reinforcements can leave too few defenders in the attackers' territory, allowing a counter-invasion from other areas.


There are many different methods by which an invasion can take place, each method having arguments both in their favour and against. These include invasion by land, sea, or air, or any combination of these methods.

Invasion by land

The Battle of Moscow during Operation Barbarossa.
The Battle of Moscow during Operation Barbarossa.

Invasion over land is the straightforward entry of armed forces into an area using existing land connections, usually crossing borders or otherwise defined zones, such as a demilitarized zone, overwhelming defensive emplacements and structures. Although this tactic often results in a quick victory, troop movements are relatively slow and subject to disruption by terrain and weather. Furthermore, it is hard to conceal plans for this method of invasion, as most geopolitical entities take defensive positions in areas that are most vulnerable to the methods mentioned above.

In modern warfare, invasion by land often takes place after, or sometimes during, attacks on the target by other means. Air strikes and cruise missiles launched from ships at sea are a common method of "softening" the target. Other, more subtle, preparations may involve secretly garnering popular support, assassinating potentially threatening political or military figures, and closing off supply lines where they cross into neighboring countries. In some cases, those other means of attack eliminate the need for ground assault; the 1945 bombing of Japan ultimately made it unnecessary to invade the main islands with infantry troops. In cases such as this, while some ground troops are still needed to occupy the conquered territory, they are allowed to enter under the terms of a treaty and as such are no longer invaders. As unmanned, long-range combat evolves, the instances of basic overland invasion become fewer; often the conventional fighting is effectively over before the infantry arrives in the role of peacekeepers (see " Applications in fourth generation warfare" in this article).

Invasion by sea

An LCAC carrying armored vehicles ashore during the invasion of Iraq.
An LCAC carrying armored vehicles ashore during the invasion of Iraq.

Invasion by sea is the use of a body of water to facilitate the entry of armed forces into an area, often a landmass adjoining the body of water or an island. This is generally used either in conjunction with another method of invasion, and especially before the invention of flight, for cases in which there is no other method to enter the territory in question. Arguments in favour of this method usually consist of the ability to perform a surprise attack from sea, or that naval defenses of the area in question are inadequate to repel such an attack. However, the large amount of specialized equipment, such as amphibious vehicles and the difficulty of establishing defenses—usually with a resulting high casualty count—in exchange for a relatively small gain, are often used as arguments against such an invasion method. Underwater hazards and a lack of good cover are very common problems during invasions from the sea. At the Battle of Tarawa, Marine landing craft became hung up on a coral reef and were shelled from the beach. Other landers were sunk before they could reach the shore, and the tanks they were carrying were stranded in the water. Most of the few survivors of the first wave ended up pinned down on the beach. The island was conquered but at a heavy cost, and the loss of life sparked mass protests from civilians in the United States.

Invasion by air

Thousands of paratroopers descend during Operation Market Garden.
Thousands of paratroopers descend during Operation Market Garden.

Invasion by air is an invention of the 20th century and modern warfare. The idea involves sending military units into a territory by aircraft. The aircraft either land, allowing the military units to debark and attempt their objective, or the troops exit the aircraft while still in the air, using parachutes or similar devices to land in the territory being invaded. Many times air assaults have been used to pave the way for a ground- or sea-based invasion, by taking key positions deep behind enemy lines such as bridges and crossroads, but an entirely air-based invasion has never succeeded. Two immediate problems are resupply and reinforcement. A large airborne force cannot be adequately supplied without meeting up with ground forces; an airborne force too small simply places themselves into an immediate envelopment situation. Arguments in favour of this method generally relate to the ability to target specific areas that may not necessarily be easily accessible by land or sea, a greater chance of surprising the enemy and overwhelming defensive structures, and, in many cases, the need for a reduced number of forces due to the element of surprise. Arguments against this method typically involve capacity to perform such an invasion—such as the sheer number of planes that would be needed to carry a sufficient number of troops—and the need for a high level of intelligence in order for the invasion to be successful.

The closest examples to a true air invasion are the Battle of Crete, Operation Thursday (the Chindits second operation during the Burma Campaign) and Operation Market Garden. Operation Market Garden was an assault on the German-occupied Netherlands conducted in September of 1944. Nearly 35,000 men were dropped by parachute and glider into enemy territory in an attempt to capture bridges from the Germans and make way for the Allies' advance. However, even with such a massive force taking the Germans completely by surprise, the assault was a tactical failure and after 9 days of fighting the Allies managed only to escape back to their own lines, having sustained over 18,000 casualties. In the 21st century, as vast improvements are made in anti-aircraft defenses, it seems that the air invasion is a strategy whose time may never come.


US forces distribute information on the streets of Kut, Iraq.
US forces distribute information on the streets of Kut, Iraq.

Once political boundaries and military lines have been breached, pacification of the region is the final, and arguably the most important, goal of the invading force. After the defeat of the regular military, or when one is lacking, continued opposition to an invasion often comes from civilian or paramilitary resistance movements. Complete pacification of an occupied country can be difficult, and usually impossible, but popular support is vital to the success of any invasion.

Media propaganda such as leaflets, books, and radio broadcasts, can be used to encourage resistance fighters to surrender and to dissuade others from joining their cause. Pacification, often referred to as "the winning of hearts and minds", reduces the desire for civilians to take up resistance. This may be accomplished through reeducation, allowing conquered citizens to participate in their government, or, especially in impoverished or besieged areas, simply by providing food, water, and shelter. Sometimes displays of military might are used; invading forces may assemble and parade through the streets of conquered towns, attempting to demonstrate the futility of any further fighting. These displays may also include public executions of enemy soldiers, resistance fighters, and other conspirators. Particularly in antiquity, the death or imprisonment of a popular leader was sometimes enough to bring about a quick surrender. However, this has often had the unintended effect of creating martyrs around which popular resistance can rally. An example of which was Sir William Wallace, who, centuries after his execution by the English, is still a symbol of Scottish nationalism.

Many factors need to be taken into account when deciding which tactics to use during occupation and, when the wrong decisions are made, it can lead to years (or even centuries) of continued resistance. The problems caused by continued resistance may be minimal if the conquered territory is only needed for a short-term tactical purpose, but can become extremely difficult if the intent is to colonize the area or hold the land indefinitely.



Without a steady flow of supplies, an invading force will soon find itself retreating. Before his invasion of Greece, Xerxes I spent three years amassing supplies from all over Asia; Herodotus wrote that the Persian army was so large it "drank the rivers dry".

In most invasions, even in modern times, many fresh supplies are gathered from the invaded territories themselves. Before the laws of war, invaders often relied heavily on the supplies they would win by conquering towns along the way. During the Second Punic War, for example, Hannibal diverted his army to conquer cities simply to gather supplies; his strategy in crossing the alps necessitated traveling with as few provisions as possible, expecting the Roman stores to sustain them when they had breached the border. The scorched earth tactics used in Russia forced Napoleon to withdraw his forces due to lack of food and shelter. Today, the Law of land warfare forbids looting and the confiscation of private property, but local supplies, particularly perishables, are still purchased when possible for use by occupying forces, and airplanes often use parachutes to drop supplies to besieged forces. Even as rules become more strict, the necessities of war become more numerous; in addition to food, shelter, and ammunition, today's militaries require fuel, batteries, spare mechanical parts, electronic equipment, and many other things. In the United States, the Defense Logistics Agency employs over 22,000 civilians with the sole task of logistics support, and 30,000 soldiers graduate from the U.S. Army Logistics Management College each year.


A mobile satellite communications center.
A mobile satellite communications centre.

Another consideration is the importance of leadership being able to communicate with the invasion force. In ancient times, this often meant that a king needed to lead his armies in person to be certain his commands were followed, as in the case of Alexander the Great. At that time, the skills needed to lead troops in battle were as important as the skills needed to run a country during peacetime. When it was necessary for the king to be elsewhere, messengers would relay updates back to the rear, often on horseback or, in cases such as the Battle of Marathon, with swift runners.

When possible, sloops and cutters were used to relay information by sea. The HMS Pickle brought Britain the first news that Nelson had defeated the French forces at the Battle of Trafalgar.

The development of Morse Code, and later voice communications by radio and satellite, have allowed even small units of skirmishers to remain in contact with the larger invasion force, to verify orders or call for artillery support and air strikes. These communications were critical to the German blitzkrieg strategy, as infantry commanders relayed defensive positions to tanks and bombers.

Applications regarding non-state combatants

In the 20th and 21st century, questions arose regarding the effectiveness of the invasion strategy in neutralising non-state combatants, a type of warfare sometimes referred to — especially in the US — as ' fourth generation warfare'. In this case, one or more combatant groups are controlled not by a centralized state government but by independent leadership, and these groups may be made up of civilians, foreign agents, mercenaries, politicians, religious leaders, and members of the regular military. These groups act in smaller numbers, are not confined by borders, and do not necessarily depend on the direct support of the state. Groups such as these are not easily defeated by straightforward invasion, or even constant occupation; the country's regular army may be defeated, the government may be replaced, but asymmetric warfare on the part of these groups can be continued indefinitely. Because regular armed forces units do not have the flexibility and independence of small covert cells, many believe that the concept of a powerful occupying force actually creates a disadvantage.

An opposing theory holds that, in response to extremist ideology and unjust governments an invasion can change the government and reeducate the people making prolonged reistance unlikely and averting future violence. This theory acknowledges that these changes may take time—generations, in some cases—but holds that immediate benefits may still be won by reducing membership in, and choking the supply lines of, these covert cells. Proponents of the invasion strategy in such conflicts maintain the belief that a strong occupying force can still succeed in its goals on a tactical level, building upon numerous small victories, similar to a war of attrition.

Contemporary debate on this issue is still fresh; neither side can claim to know for certain which strategies will ultimately be effective in defeating non-state combatants. Opponents of the invasion strategy point to a lack of examples in which occupying or peacekeeping forces have met with conclusive success. They also cite continuing conflicts such as Northern Ireland, Israel, Chechnya, and Iraq, as well as examples which they claim ultimately proved to be failures, such as Lebanon, and Afghanistan. Supporters of the invasion strategy hold that it is too soon to call those situations failures, and that patience is needed to see the plan through. Some say that the invasions themselves have, in fact, been successful, but that political opponents and the international media skew the facts for sensationalism or political gain.


The outcomes of an invasion may vary according to the objectives of both invaders and defenders, the success of the invasion and the defense, and the presence or absence of an agreed settlement between the warring parties. The most common outcome is the loss of territory, generally accompanied by a change in government and often the loss of direct control of that government by the losing faction. This sometimes results in the transformation of that country into a client state, often accompanied by requirements to pay reparations or tribute to the victor. In rare cases the results of a successful invasion may simply be a return to the status quo; this can be seen in wars of attrition, when the destruction of personnel and supplies is the main strategic objective.

Record-setting invasions

Many records for invasions were set during World War II, at the peak of second and third generation warfare. The vast numbers of the armies involved combined with innovative tactics and technology lent themselves, arguably for the last time in history, to invasions on a massive scale.

The largest invasion in history was 1941's Operation Barbarossa, in which 4,000,000 German troops blitzkrieged into Russia. The Germans advanced with ease at first and nearly captured Moscow, also laying siege to Leningrad, but soon found themselves fighting the harsh Russian winter and fierce Soviet resistance, and their advance ground to a halt at Stalingrad in early 1943.

In the largest amphibious invasion in history, 156,215 Allied troops landed at Normandy to retake France from the occupying German forces commanded by Rommel. Though it was costly in terms of men and materials, the invasion advanced the Western front and forced Germany to redirect its forces from the Russian and Italian fronts. In hindsight, it is also credited with defining the Western boundary of Soviet communism; had the Allies not advanced, it is conceivable that the Soviet Union would have controlled even more of Europe than they eventually did.

Other examples of historically significant invasions

Assyrian invasion of the Kingdom of Israel

Sargon II, during the course of conquering much of what is now known as the Middle East, defeated the Kingdom of Israel in 722 BC and sent its inhabitants into exile. This presaged future Greek and Roman conquest and, later, the Crusades. To this day, the region remains contested.

Persian invasion of Greece

In 480 BC, Xerxes I of Persia moved his armies against the loose confederation of city-states in what is modern-day Greece. One of the most famous battles of the war, fought at Thermopylae, is an early example of using a chokepoint to tactical advantage. Although Xerxes' army was vast—modern estimates put it at 250,000—the defending Greeks were able to hold their ground for days by using a narrow mountain pass to slow the Persian advance. The invasion also demonstrates the importance of communication and supply routes; although Xerxes' land battles were almost all Persian victories, the Greeks managed to cut off his naval support and the Persians were forced to withdraw. The invasion served to unify the various city-states, bringing about the formation of the Greek nation.

Macedonian conquest of the Persian Empire

In 323 BC, Alexander the Great led his army into Persia, defeating Darius III, conquering Babylon, and taking control of the Persian Empire. Alexander's influence in mixing cultures led to the Hellenistic Age of Mesopotamia and North Africa.

The Muslim Conquests

Following Muhammad's unification of the Arabian peninsula in 632, his successors in the Caliphate began a series of invasions of the Middle East, North Africa, Southern Europe, and South Asia. Lasting slighly more than a century, these conquests brought much of the ancient world under the banner of Islam, and instituted a scientific renaissance as well as a cultural and societal revolutions in these regions.

The Norman Invasion of England

The 1066 invasion of England by William the Conqueror, and the decisive battle which won the war, the Battle of Hastings, were to have profound effects on the historical and societal development of Britain, and the English language itself.

The Crusades

In a series of nine different major invasions, from 1095 to 1291, the Catholic Church attempted to conquer the Holy Land from its Muslim rulers, with varied success. As Jerusalem changed hands and European forces moved back and forth, in-roads to the Levant were reestablished and the cultures mixed on a large scale for the first time in centuries.

Genghis Khan’s invasions of China

From 1206 until his death in 1227, Genghis Khan orchestrated a series of invasions that united much of Asia. Relying heavily on cavalry, the Mongol hordes were able to travel quickly yet were well-supplied. By 1368, the Mongol Empire was the largest contiguous empire in history, comprised of 35 million km² (13.8 million miles²) of territory stretched across the continent. His eastward invasion of China created the Yuan Dynasty, and his westward invasion of Kiev Rus further linked Europe and Asia by reestablishing the Silk Road.

Conquest of the Aztec Empire

The last of the Aztec empire was destroyed at Tenochtitlan in 1521, by a combination of Spanish and native forces. Aided by 2,000 local Tlaxcalan warriors, Hernán Cortés marched into the city. Although he and his men were expelled, they returned with ships and laid siege to the capital. Though an epidemic of smallpox took its toll on the Aztecs, Cortes' conquest was the culmination of Spanish strategy in the Americas: He used promises to gain native allies, he burned and killed thousands of people to strike fear into his enemies, and he combined superior technology with patience while he struck at Tenochtitlan from the sea. This opened the door to Spanish colonization of mainland Mesoamerican cultures.

French invasion of Russia

In 1812, Napoleon led his Grande Armée into Russia. At that point, his invasion force of 691,500 men was the largest ever assembled, and for several weeks the Russian Army could do nothing but retreat and try to buy time. The first major battle between the two armies, at the Russian defenses of Borodino, was one of the bloodiest single days in human history, with estimates of at least 65,000 dead. But although the Russian retreat allowed the French to capture Moscow, they were left depleted and without shelter or supplies. Napoleon was forced to withdraw. Although this invasion was not the end of Napoleon, it is credited with fostering a powerful patriotism in Russia that would lead to the strengthening of the nation in the 19th and 20th centuries.

Christian Colonialism and Imperialism

In the 15th century the Christian Nations of European began the modern age of colonialism with the " Age of Discovery", led by the Spanish colonization of the Americas and Portuguese Empire in the Americas and along the coasts of Africa, the Middle East, India, and East Asia. The Roman Catholic Church played a large role in their overseas activities and the enormous trade profits and riches from gold and silver mines allowed them to finance costly religious wars in Europe. During the 16th and 17th centuries, England, France and Holland established their own overseas empires in direct competition with each other as well as the earlier Iberian ones, while the land based Russian Empire expanded across northern and central asia. These activities resulted in the invasions of the Indian subcontinent to set up the extensive European colonies in India, as well as the invasion of Africa called the Scramble for Africa and the colonization of the East Indies. Towards the 17th and 18th century the Germans and the French also joined in, beginning the 3rd wave of invasions that would subdue native peoples and economines, and expand European controlled territory over the majority of the globe.

Decolonization began in the 19th century and picked up pace only after World War II left the European empires weakened and struggling to subdue the native resistance across the vast expanses of their empires. Debates upon the negative vs. positive impact and evaluation of colonialism and colonization- such as those of colonial Christianization, genocide, third world debt and slavery- upon the colonizer and the colonized continue to shape global and national polotics to this day.

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