guerrilla warfare vs irregular warfare

guerrilla warfare

noun
  • Combat using groups of irregular, light, mobile troops within areas occupied by an enemy force, usually through hit-and-run tactics. 

irregular warfare

noun
  • 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. 

How often have the words guerrilla warfare and irregular warfare occurred in a corpus of books? (source: Google Ngram Viewer )