In his book, Mao describes guerrilla warfare as one of many methods used by an oppressed people to combat aggression. Mao divides guerrilla warfare into three phases. Phase I is devoted to the organization of an underground resistance movement to spread propaganda and elicit support for the movement from the people.The purpose of the underground is to develop support for the overthrow of the existing government, or for resistance against an occupying force. The underground does not get involved in direct military action against the enemy, but harasses the enemy through espionage, sabotage, or civil unrest. Based on the basic strategy and tactics of warfare as described by Sun-tzu, Mao stresses the importance of guerrilla warfare tactics in a revolutionary war, emphasizing that they must be combined in conjunction with conventional warfare tactics.