Asymmetric warfare
Definition and differences: The popularity of the term dates from Andrew J. R. Mack's 1975 article "Why Big Nations Lose Small Wars" in World Politics, in which "asymmetric" referred simply to a significant disparity in power between opposing actors in a conflict. "Power," in this sense, is broadly understood to mean material power, such as a large army, sophisticated weapons, an advanced economy, and so on. Mack's analysis was largely ignored in its day, but the end of the Cold War sparked renewed interest among academics. By the late 1990s, a new research building off Mack's works was beginning to mature; after 2004, the U.S. military began once again to prioritize responding to challenges presented by asymmetric warfare.
Since 2004, the discussion of asymmetric warfare has been complicated by the tendency of academic and military officials to use the term in different ways, as well as by its close association with guerrilla warfare, insurgency, terrorism, counterinsurgency, and counterterrorism.
Academic authors tend to focus on explaining two puzzles in asymmetric conflict. First, if "power" determines victory, there must be reasons why weaker actors decide to fight more powerful actors. Key explanations include:

Weaker actors may have secret weapons.
Weaker actors may have powerful allies.
Stronger actors are unable to make threats credible.
The demands of a stronger actor are extreme.
The weaker actor must consider its regional rivals when responding to threats from powerful actors.
Second, if "power," as generally understood, leads to victory in war, then there must be an explanation for why the "weak" can defeat the "strong." Key explanations include:

Strategic interaction.
Willingness of the weak to suffer more or bear higher costs.
External support of weak actors.
Reluctance to escalating violence on the part of strong actors.
Internal group dynamics.
Inflated strong actor war aims.
Evolution of asymmetric rivals' attitudes towards time.
Asymmetric conflicts include interstate and civil wars, and over the past two hundred years, have generally been won by strong actors. Since 1950, however, weak actors have won the majority of asymmetric conflicts. In asymmetric conflicts conflict escalation can be rational for one side.

Strategic basis: In most conventional warfare, the belligerents deploy forces of a similar type, and the outcome can be predicted by the quantity or quality of the opposing forces, for example, better command and control of theirs (c2). There are times when this is the case, and conventional forces are not easily compared, making it difficult for opposing sides to engage. An example of this is the standoff between the continental land forces of the French Army and the maritime forces of the United Kingdom's Royal Navy during the French Revolutionary and Napoleonic Wars. In the words of Admiral Jervis during the campaigns of 1801, "I do not say, my Lords, that the French will not come. I say only they will not come by sea", and a confrontation that Napoleon Bonaparte described as that between the elephant and the whale.

Tactical basis: The tactical success of asymmetric warfare is dependent on at least some of the following assumptions:

One side can have a technological advantage that outweighs the numerical advantage of the enemy; the English longbow at the Battle of Crécy is an example.
Technological superiority usually is cancelled by the more vulnerable infrastructure, which can be targeted with devastating results. Destruction of multiple electric lines, roads, or water supply systems in highly populated areas could devastate the economy and morale. In contrast, the weaker side may not have these structures at all.
Training, tactics, and technology can prove decisive and allow a smaller force to overcome a much larger one. For example, for several centuries, the Greek hoplite's (heavy infantry) use of phalanx made them far superior to their enemies. The Battle of Thermopylae, which also involved good use of terrain, is a well-known example.
If the inferior power is in a position of self-defense, i.e., under attack or occupation, it may be possible to use unconventional tactics, such as hit-and-run and selective battles in which the superior power is weaker, as an effective means of harassment without violating the laws of war. Perhaps the classic historical examples of this doctrine may be found in the American Revolutionary War, movements in World War II, such as the French Resistance and Soviet and Yugoslav partisans. Against democratic aggressor nations, this strategy can be used to play on the electorate's patience with the conflict (as in the Vietnam War, and others since), provoking protests, and consequent disputes among elected legislators.
However, if the weaker power is in an aggressive position or turns to tactics prohibited by the laws of war (jus in bello), its success depends on the superior power's refraining from like tactics. For example, the law of land warfare prohibits the use of a flag of truce or marked medical vehicles as cover for an attack or ambush. Still, an asymmetric combatant using this prohibited tactic to its advantage depends on the superior power's obedience to the corresponding law. Similarly, warfare laws prohibit combatants from using civilian settlements, populations or facilities as military bases, but when an inferior force uses this tactic, it depends on the premise that the superior one will respect the law that the other is violating, and will not attack that civilian target, or if they do the propaganda advantage will outweigh the material loss.

Terrorism: There are two opposing viewpoints on the relationship between asymmetric warfare and terrorism. In the modern context, asymmetric warfare is increasingly considered a component of fourth generation warfare. When practiced outside the laws of war, it is often defined as terrorism, though rarely by its practitioners or their supporters. The other view is that asymmetric warfare does not coincide with terrorism.

Use of terrain: Terrain that limits mobility, such as forests and mountains, can be used as a force multiplier by the smaller force and as a force inhibitor against the larger one, especially one operating far from its logistical base. Such terrain is called difficult terrain. Urban areas, though generally having good transport access, provide innumerable ready-made defensible positions with simple escape routes and can also become rough terrain if prolonged combat fills the streets with rubble:

The contour of the land is an aid to the army, sizing up opponents to determine victory and assessing dangers and distance. "Those who do battle without knowing these will lose."
The guerrillas must move amongst the people as a fish swims in the sea.
In the 12th century, irregulars known as the Assassins were successful in the Nizari Ismaili state. The "state" consisted of fortresses (such as the Alamut Castle) built on strategic mountaintops and highlands with difficult access, surrounded by hostile lands. The Assassins developed tactics to eliminate high-value targets, threatening their security, including the Crusaders.
In the American Revolutionary War, Patriot Lieutenant Colonel Francis Marion, known as the "Swamp Fox," took advantage of irregular tactics, interior lines, and the wilderness of colonial South Carolina to hinder larger British regular forces.
Yugoslav Partisans, starting as small detachments around mountain villages in 1941, fought the German and other Axis occupation forces, successfully taking advantage of the rough terrain to survive despite their small numbers. Over the next four years, they slowly forced their enemies back, recovering population centers and resources, eventually growing into the regular Yugoslav Army.

Role of civilians: Civilians can play a vital role in determining the outcome of an asymmetric war. In such conflicts, when it is easy for insurgents to assimilate into the population quickly after an attack, tips on the timing or location of insurgent activity can severely undermine the resistance. An information-central framework, in which civilians are seen primarily as sources of strategic information rather than resources, provides a paradigm to understand better the dynamics of such conflicts where civilian information-sharing is vital. The framework assumes that:

The consequential action of non-combatants (civilians) is information sharing rather than supplying resources, recruits, or shelter to combatants.
Information can be shared anonymously without endangering the civilian who relays it.
Given the additional assumption that the larger or dominant force is the government, the framework suggests the following implications:

Civilians receive services from government and rebel forces as an incentive to share valuable information.
Rebel violence can be reduced if the government provides services.
Provision of security and services are complementary in reducing violence.
Civilian casualties reduce civilian support to the perpetrating group.
Provision of information is strongly correlated with the level of anonymity that can be ensured.
A survey of the empirical literature on conflict, does not provide conclusive evidence on the claims. But the framework gives a starting point to explore the role of civilian information sharing in asymmetric warfare.

War by proxy: Where asymmetric warfare is carried out (generally covertly) by allegedly non-governmental actors who are connected to or sympathetic to a particular nation's (the "state actor's") interest, it may be deemed war by proxy. This is typically done to give the state actor deniability. The deniability can be crucial to keep the state actor from being tainted by the actions, to allow the state actor to negotiate in apparent good faith by claiming they are not responsible for the actions of parties who are merely sympathizers, or to avoid being accused of belligerent actions or war crimes. If proof emerges of the true extent of the state actor's involvement, this strategy can backfire; for example, see Iran-contra and Philip Agee.

Examples: 

American Indian Wars: Benjamin Church designed his force primarily to emulate Native American patterns of war. Toward this end, Church endeavored to learn to fight like Native Americans from Native Americans.