In American libertarian ideology, the holy grail ethical concept is the non-aggression principle, which roughly states that no one may engage in or threaten aggression against others (and their property), often quickly qualified with the idea that defensive force may be used in the face of aggression. When stated in such a simple and abstract way, it is something that is hard for very many people to particularly disagree with, and because of this it also makes it such that it can be used to deceive people into thinking they might be libertarians.
But if you don’t just take it at face value and continue to talk to a libertarian about aggression, it will become clear that they have some specific notion of where the line is between “aggression” and “defense”, as most people do. And that notion is informed by other ideas and principles, by their analysis of society, and by the personality of the given libertarian. The way a person responds to different scenarios in what they consider justified and unjustified force will depend on these kind of factors.
Whatever particular interpretation the libertarian gives is functionally one version of the non-aggression principle out of many. The reality is that virtually everyone who is not either a pacifist, psychopath, or totalitarian believes in something resembling the NAP, and therefore there isn’t necessarily anything novel about it. Liberals, conservatives, socialists and libertarians alike. Different political views just define the social context of aggression differently, draw the aggression/defense line in different places, have different analysis of society.
Libertarians themselves have gotten into many heated internal debates over how this stuff is hashed out (different interpretations of property theory, minarchism vs anarchism, and so on). And for libertarians, how aggression gets defined in practice is highly dependent on their ideas about property and markets, and the principle is used as a rhetorical tool to push one into adopting a consistent position opposing the state. On the flip side, in the eyes of many of those who disagree with libertarians, there is aggression involved with upholding their ideas about property and markets.
The NAP is smoked and mirrors for what is a matter of debate about the proper use of force and how it is organized. Radical American libertarians frequently advocate for essentially privatizing the mechanisms of force in society – they are not against aggression as much as they are against aggression in the hands of public or quasi-public institutions. Anarcho-capitalists do this in the most blatant way by advocating private police and defense. Paleo-libertarians and libertarian racialists advocate aggression at least implicitly for the restructuring of society involved in what they propose to be enforced – the thought of all the forced evictions is horrifying. All of this stuff hinges on the ideas about property and the market. Non-compliance to the libertarian’s property regime merits aggression.
While libertarians might think of the NAP as a moral shroud, the emperor has no clothes. Many libertarians support aggression, either explicitly or implicitly, whether knowingly or not. Their philosophy is primarily about property and economic power, not non-aggression. For the libertarian in their propagandizing mode in discourse, the NAP is a rhetorical tactic to moralize one’s political opponents – “See, you support putting guns to people’s heads! You should be ethically consistent and oppose the state” or “you disagree about property? you support aggression!”.
This frequently ends up being a sensationalist pile of muck in which the libertarian cries aggression in even the most ridiculous of scenarios, parading around like “don’t you tell me what to do!” at the thought of property owners and market actors being held to reasonable ethical standards. The libertarian sees aggression in every disagreement with their notions of property and their policy ideas about the market, while being blinded to the aggression implicit in them. The NAP just begs the question and conceals the violence within libertarianism.