Guerrilla warfare
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A guerrilla is when civilians attack a regular Army. When civilians attack civilians is usually called terrorism. When two or more armies (representing countries) participate in a military conflict this is a war.
Guerrilla warfare is a war tactic which involves people (called guerrilleros) using the land and geography as an advantage against an organized army. Usually this army is invading a territory.
Guerrilla is a word of Spanish origin. It means "little war".