The term 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.
Military operations that occur within the territory of a single geopolitical entity can sometimes be termed an invasion 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 the other faction in a civil war or insurrection situation. For example, during both the American Revolutionary War and the American Civil War many of the military operations conducted during these wars are called invasions for this reason even though they did not involve "foreign" armies entering from "foreign" nations strictly speaking.
It should be noted that the term invasion does not, in and of itself, imply either a justified or unjustified course of action. For example, during World War II, German military operations conducted against Poland in 1939 are often called the Invasion of Poland while military operations conducted against Nazi controlled France in 1944 is called the Invasion of Normandy. Both military operations are properly called invasions because they involved an outside force entering territory not under its authority or control at the time. The morality or immorality of the military operation itself is not a factor in determining whether it is termed as an invasion.
States with potentially hostile neighbors 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, 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.
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. The obsolescence of large fortifications was displayed by the failure of the Maginot Line in the beginning of World War Two. 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 intended to intercept 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 defense force.
In particularly large nations, the defending force may also retreat in order to facilitate a counterattack 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 Soviets 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, as happened in the Second Punic War.
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 atomic-bombing of Hiroshima and Nagasaki ultimately made it unnecessary for the Allies to invade the Japanese home 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 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 favor 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 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 favor 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. The latter was an assault on the German-occupied Netherlands conducted in September 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.
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; 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.
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 stricter, 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.
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.
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.
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 resistance 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 largest land invasion in history was 1941's Operation Barbarossa, in which 4,000,000 German troops blitzkrieged into the Soviet Union. Initially the Germans advanced with great ease and nearly captured Moscow, also laying siege to Leningrad, but soon found themselves fighting the harsh Russian winter as well as stiffer 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. 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, the operation 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 more of Europe than it eventually did.
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, slavery, abolition of slavery, infrastructures and medical advances —upon the colonizer and the colonized continue to shape global and national politics to this day.European Theatre of World War II encompassed the conflict in central and eastern Europe from June 22, 1941 to May 9, 1945. It was the largest theatre of war in history in terms of numbers of soldiers, equipment and casualties and was notorious for its unprecedented ferocity, destruction, and immense loss of life. The fighting involved millions of German and Soviet troops along a broad front hundreds of kilometres long. It was by far the deadliest single theatre of World War II. Scholars now believe that as many as 27 million Soviet citizens died during the war, including some 8.7 million soldiers who fell in battle against Hitler's armies or died in POW camps. Millions of civilians died from starvation, exposure, atrocities, and massacres.
The USSR, which had grafted onto the Russian Soviet Federative Socialist Republic several countries that had had short-lived independence (Ukraine, Georgia, Armenia, Azerbaijan, and the lands of Central Asia), never reconciled itself to having lost West Ukraine, West Belarus, Bessarabia, and the three Baltic states (territories which formerly belonged to the Russian Empire) in the course of 1919-21. Thus they aimed to annex these territories as well as to obtain a buffer zone from Finland in 1939-40 (see Soviet-Finnish War). After the Soviet invasion of Poland following the corresponding German invasion that marked the start of World War II in 1939, the Soviet Union annexed eastern parts (so-called "Kresy") of the Second Polish Republic (see Molotov-Ribbentrop Pact). In 1940 the Soviet Union annexed Estonia, Latvia, Lithuania, Bessarabia and Bukovina (see Occupation of Baltic states).
During the Cold War, the term Eastern Bloc (or Soviet Bloc) was used to refer to the Soviet Union and countries it controlled in Central and Eastern Europe (Bulgaria, Czechoslovakia, East Germany, Hungary, Poland, Romania). Hungary was invaded by the Soviet Army in 1956 after it had overthrown its pro-Soviet government and replaced it with one that sought a more democratic communist path independent of Moscow; when Polish communist leaders tried to elect Władysław Gomułka as First Secretary they were issued an ultimatum by Soviet military that occupied Poland ordering them to withdraw election of Gomulka for the First Secretary or be "crushed by Soviet tanks". Czechoslovakia was invaded in 1968 after a period of liberalization known as the Prague Spring. The latter invasion was codified in formal Soviet policy as the Brezhnev Doctrine. In 1979 the Soviet Union invaded Afghanistan to ensure that a pro-Soviet regime would be in power in the country (see Soviet war in Afghanistan). Poland was afaid of getting invaded by the USSR during the early 1980s due to the stirkes assosiated with solidarity which called for an end to the communist government there.