irregular warfare

Short definition from Wiktionary

Noun

1. 1
   Warfare in which one or more combatants are irregulars (such as partisans, guerrillas or insurgents) rather than regular military forces.
2. 2
   Warfare in which one or more combatants are irregulars (such as partisans, guerrillas or insurgents) rather than regular military forces: Warfare with the aim of gaining or establishing political authority using asymmetric warfare tactics.