The Non-Aggression Principle [N.A.P.] formalizes a way of living that many people already believe in:
No one has the right, under any circumstances, to initiate force against another human being, nor to delegate its initiation.-- L. Neil Smith
[nonaggression.org]
The fundamental axiom of libertarian theory is that no one may threaten or commit violence ('aggress') against another man's person or property. Violence may be employed only against the man who commits such violence; that is, only defensively against the aggressive violence of another. In short, no violence may be employed against a nonaggressor. Here is the fundamental rule from which can be deduced the entire corpus of libertarian theory.
Murray N. Rothbard,
"War, Peace, and the State,"
The Myth of National Defense
[rothbard]
We shall not attempt to justify this axiom here: Most libertarians and even conservatives are familiar with the rule and even defend it; the problem is not so much in arriving at the rule as in fearlessly and consistently pursuing its numerous and often astounding implications.
Murray N. Rothbard,
"War, Peace, and the State,"
The Myth of National Defense
[rothbard]