A guerrilla is a civilian who attacks a regular army.
Guerrilla warfare is a war tactic in which people (Spanish: guerrilleros) fight against an organized army. Guerrilla warfare is sometimes practiced in places in which a regular army would have difficulty, such as forests and mountains. Usually, the army is invading a territory. In open fields, the organized army, which is better armed and larger, has the advantage. However, in forests and mountains, the guerillas can gain an advantage over larger and better-armed armies. In some wars such as the American Revolutionary War or the Vietnam War, guerilla warfare tactics have been successful and were one of the main reasons (but not the only one) for their victory.
Guerrilla, is a word of Spanish origin, and means "little war". It was first used in 1808, when Spain was invaded by Napoleon, resulting in the Peninsular War with Spanish guerilla forces resisting the French Army.
This war tactic was used in British America by the Native Americans. In various wars, the natives fought guerilla wars for the French or the British or against them or other enemies. During the American Revolutionary War, the Rebels and Loyalists used guerilla warfare against enemy regular forces. Continuing the pre-war fighting of Bleeding Kansas, guerilla warfare was common in the Border States during the American Civil War. It was used also during the Vietnam War by the North Vietnamese against the French.