Asymmetric warfare originally referred to war between two or more actors or groups whose relative military power differs significantly. Contemporary military thinkers tend to broaden this to include asymmetry of strategy or tactics; today "asymmetric warfare" can describe a conflict in which the resources of two belligerents differ in essence and in the struggle, interact and attempt to exploit each other's characteristic weaknesses. Such struggles often involve strategies and tactics of unconventional warfare, the "weaker" combatants attempting to use strategy to offset deficiencies in quantity or quality.

Typically at least one of the parties involved may be referred to as partisans. One theory says "partisan" comes from the Tuscan word, "partigiano", meaning a member of a party of light or irregular troops engaged in harassing an enemy, especially a member of a guerrilla band engaged in fighting or sabotage against an occupying army. The other theory says the word comes from the Persian word Partisan; in Persian, san means similar and Part is a name of an Aryan tribe living in the northeast of Iran who are said to have invented and developed the first partisan war tactics.

The first known wide usage of asymmetric war was by Parthians, who freed Persia from Seleucid rule (remaining from Alexander's invasion) and continued the same techniques against Romans and other invaders from the north of the empire.

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

• One side can have a technological advantage which outweighs the numerical advantage of the enemy; the decisive English Longbow at the Battle of Agincourt is an example. The advantage may also be the other way around. For example, the vast numerical superiority of the Chinese forces during their initial involvement in the Korean War overwhelmed the technological superiority of the United Nations forces.

• Training and tactics as well as 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 classical historical examples of this doctrine may be found in the American Revolutionary War and 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.

• If the inferior power is in an aggressive position, however, and/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 clearly-marked medical vehicles as cover for an attack or ambush, but an asymmetric combatant using this prohibited tactic to its advantage depends on the superior power's obedience to the corresponding law. Similarly, laws of warfare prohibit combatants from using civilian settlements, populations or facilities as military bases, but when an inferior power uses this tactic, it depends on the premise that the superior power 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. As seen in most conflicts of the 20th and 21st centuries, this is highly unlikely as the propaganda advantage has always outweighed adherence to international law, especially by dominating sides of any conflict.