Unconventional warfare (UW) is broadly defined as "military and quasi-military operations other than conventional warfare" and may use covert forces, subversion, or guerrilla warfare. This is typically done to avoid escalation into conventional warfare as well as international conventions.
Aside from the earlier definition of warfare that is not conventional, unconventional warfare has also been described as:
There is another type of warfare— new in its intensity, ancient in its origin—war by guerrillas, subversives, insurgents, assassins; war by ambush instead of by combat, by infiltration instead of aggression, seeking victory by eroding and exhausting the enemy instead of engaging him. It preys on unrest.
Unconventional warfare targets the civilian population psychologically to win hearts and minds, and only targets military and political bodies for that purpose, seeking to render the military proficiency of the enemy irrelevant. Limited conventional warfare tactics can be used unconventionally to demonstrate might and power, rather than to reduce the enemy's ability to fight substantially. In addition to the surgical application of traditional weapons, other armaments that specifically target the military can be used are: airstrikes, nuclear weapons, incendiary devices, or other such weapons.
Special Forces, inserted deep behind enemy lines, are used unconventionally to train, equip, and advise locals who oppose their government. They can also spread subversion and propaganda, while they aid native resistance fighters, to ultimately cause a hostile government to capitulate. Tactics focus on destroying military targets while avoiding damage to civilian infrastructure and blockading military resupply are used to decrease the morale of government forces.
The USA Department of Defense defines unconventional warfare as activities conducted to enable a resistance movement or insurgency to coerce, disrupt, or overthrow a government or occupying power by operating through or with an underground, auxiliary, and guerrilla force in a denied area.