The phrase you're referring to is often associated with the warning given to Colonial militias during the American Revolutionary War, particularly at the battles of Lexington and Concord. The instruction was typically understood as "not to fire unless fired upon."
This directive underscored the Colonists' desire to avoid unnecessary conflict and to position themselves as defenders rather than aggressors in the escalating tensions with British forces. The focus was on self-defense and responding only to direct provocation.