Asymmetric warfare (or asymmetric engagement) is a type of war between belligerents whose relative military power, strategy or tactics differ significantly. This type of warfare often involves insurgents, terrorist groups, or resistance militias operating within territory mostly controlled by the superior force.

Asymmetrical warfare can also describe a conflict in which belligerents' material resources are uneven, and consequently, each may attempt to exploit each other's relative weaknesses. Such struggles often involve unconventional warfare, with the weaker side attempting to use strategy to offset deficiencies in the quantity or quality of their forces and equipment. Such strategies may not necessarily be militarized. This is in contrast to symmetrical warfare, where two powers have comparable military power, resources, and rely on similar tactics and victory standards.

Conventional militaries consider asymmetric warfare a form of irregular warfare – conflicts in which nominally weaker adversaries are not regular military forces of nation-states. For military analysts the term is most often used as an umbrella term to describe what is also called guerrilla warfare, insurgency, counterinsurgency, rebellion, terrorism, and counterterrorism.

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 to a significant disparity in relative 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, new research building on Mack's work was beginning to mature; after 9/11, the U.S. military began once again to grapple with asymmetric warfare strategy.

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:

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:

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.