[LSJohn]

If the perpetrator is in Afghanistan, then a libertarian government goes to Afghanistan to get him

There's nothing "imperial" about doing that. Where you'll run into trouble is trying to establish or manipulate the government of that country to your own perceived needs while or after you "get" the perpetrator[s] and there's nothing libertarian about that.

[annalex]

trying to establish or manipulate the government of that country to your own perceived needs

If that government is illegitimate, I am free to manipulate and establish whatever I want there. Things would be different if the Taliban government was representative of Afghani citizens and respectful of the rights of American citizens.