The Non-Aggression Principle (also referred to as "The NAP") is a fundamental principle that is paramount to the libertarian philosophy. It can be interpreted differently by different people. Some people believe it only refers to property and others believe it refers to the three Natural Rights (life, liberty, and property). In short, as long as you don't violate anyone's rights, you may do as you please. The second somebody's rights are violated, they have the full right to retaliate. All of the following examples take place in a fictional, libertarian society (henceforth referred to as "Ancapistan").
Person A steals Person X's car. Person X has the right to come after the thief and get his property back by any means necessary. Person X could also assign the responsibility of protecting his rights to a private security firm. Person X could pay a one-time fee to acquire his property, or he could have insurance where he pays monthly or yearly. The beauty of that system is that it's voluntary. If he's unhappy with his service, he could refuse to pay and receive services from elsewhere. Person X or Security Firm D successfully reacquires the stolen property and Person A is ostracized.
A group of 100 people get together and decide that they should form a government to rule over the 319,000,000 inhabitants of Ancapistan. If the interpretation of the NAP is the protection of life, liberty, and property, then this action violates the Principle. The free people of Ancapistan have the right to reject the newly forming government. Any force or coercion used to attempt to govern is a violation and can be defended against.
Mugger C comes running at Person A with a gun in hand with clear, violent intent. Person A absolutely has the right to defend himself from this oncoming attacker. Quickly, he pulls out his handgun and fires two rounds into Mugger C's legs. Person A was protecting his right to life.
The NAP isn't a law in the constitution of a free people. It's a naturally occurring element that is the fundamental principle in any society. Everything else is tyranny. If you want to refer to the NAP as "the law" then that's fine, as long as you understand that the "enforcers" are you and me. It is our responsibility to protect ourselves from these violations and teach one another about the prosperity the Non-Aggression Principle brings.
The NAP does not require anyone to enforce it.
It is time to raise the bar again...
The people that don't enforce it are going to get their rights violated.