The non-aggression principle is a moral principle prominent in libertarianism that states that it is immoral to initiate an act of force against a person or his property. This is usually considered the primary principle of the foundation of libertarianism, as libertarianism in general holds to the idea that anything that does not harm a person or his property is a just exercise of liberty, and is should be protected at all cost. The non-aggression principle calls for the rigorous self-defense of person and property and force can be used to counteract force, but the person abiding by this principle is never to initiate the force to begin with.
In many different schools of libertarianism, taxation is considered theft and is thus considered an unjust act of force by the state, and it is believed in these schools that the state has a monopoly on this sort of force.
The non-aggression principle is the subject of an important distinction between libertarianism, on one hand, and Biblical and movement conservative morality, in the other, since both Biblical and movement conservative morality reject it.