In libertarianism, the non-aggression principle is not a commitment to pacifism. When someone trespasses on someone else's property, the trespasser is considered the initial aggressor, and the property owner is within their right to remove the trespasser from the property.

> Consider the following hypothetical example:

>> Trespass: George is walking across a field when David jumps out from behind a bush, grabs him, and gently but forcefully removes him from the area.

> In this example, it certainly looks as though David is the one committing an act of aggression against George. After all, George is simply walking along, minding his own business, when David comes from out of nowhere, physically lays hands on him, and drags him off the field.

> But suppose that the field in this example is David’s property, and that George is knowingly trespassing across it. In that case it is clear that most libertarians would say that it is actually George who has committed aggression. David’s violence, they would say, is simply an appropriate response to George’s impermissible intrusion upon his land.

https://www.cambridge.org/core/journals/social-philosophy-an...

Libertarians oppose expropriation and eminent domain because the government is the one initiating the aggression against the property owner by forcible seizure. (Although libertarians support limiting the power of the state, it is not the same thing as eliminating the state entirely. That would be anarchism, not libertarianism.)