Much of the United State’s foreign policy revolves around the concept of preemptive war. Although the concept seems simple on paper, take out potential enemies before they have a chance to be an actual threat, it becomes ethically dubious once you start thinking about the concept.
As with most concepts we need to break this one down to the lowest level which is the individual. Let us consider the concept of preemptive war but replace countries with individual people. In our hypothetical scenario we’re going to say you live next to Ishmael whom you believe to be not quite right in the head. Ishmael often talks about his belief that the apocalypse is coming and that it is his duty to destroy the enemies of his god, Loki. You’ve been watching Ishmael stock up on enough firepower to arm a small militia but suddenly you hear whispers that he’s found somebody willing to sell him a grenade launcher. After hearing this news you realize that Ishmael with a grenade launcher may not be the best thing in the world for your life expectancy and thus feel as though you need to stop him before he does something stupid and ends up killing you. Of course you have no actual proof that Ishmael means you harm or would actually be dangerous with a grenade launcher in his possession but it’s a possibility.
Using the concept of preemptive war you would then go over to Ishmael’s house, knock on his door, and shoot him in the face when he opened it. Congratulations, you’ve just enacted United States foreign policy in your own neighborhood.
This is the problem with preemptive war, it can’t legitimately be considered an act of self-defense because there is no fear of immediate and tangible danger. Self-defense necessarily requires the defender to feel as though there is an immediate and tangible threat to their life. You can’t shoot a teenage boy on the street and later justify your action by claiming he may have mugged you if given the change.
Preemptive war isn’t a form of self-defense no matter how many times our government claims it to be. It is the initiation of violence plain and simple and that is why libertarians oppose it.
Except in the case of Iran, we have a country that has sabre-rattled for decades, directly supported terrorists and used proxies for warfare, and they have a crazy leader who is a Holocaust-denier and has publicly stated that the retaliatory destruction of Iran is acceptable losses if it can use nuclear weapons to destroy Israel.
To adapt your neighborhood example, Ishmael believes Loki will grant him paradise if he can get a hold of a grenade launcher and use it to kill you and your friendly neighbor. Indeed, he has often threatened to kill your friendly neighbor and then you and the police are useless in preventing his acquisition of the grenade launcher. Would you wait until he has it and is pointing it at your house before you decide to open fire?