From Mises Wiki, the global repository of classical-liberal thought
Force is the act of compelling someone to do (or to not do) something against her or his will.
Libertarians generally advocate one of two positions on force. Standard libertarian theory holds that force is never legitimate when employed by an aggressor, i.e., one who initiates force or fraud against the person or the justly-acquired property of others, but that there is nothing intrinsically unethical about using force defensively against an aggressor so long as the defensive force is proportional to the initiated force. Anarchopacifists like Robert LeFevre, on the other hand, hold that force is never legitimate, regardless of whether its employer is the initiator or the responder.