I thought of this question after reading through the "Stopping Dictators" thread. The question is pretty simple: What if any wars fought throughout history have been justified from a libertarian viewpoint?
...And nobody has ever taught you how to live out on the street, But now you're gonna have to get used to it...
One of the Rothbard mp3's is titled "Two Just Wars: 1776 and 1861".
The American War for Independence and War for Southern Independence* are the only two wars in American history I can think of off the top of my head fought with the furtherment of liberty in mind, where no peaceful alternatives could have been effectively used, and where no bad decisions on the behalf of our government made in the past contributed to the events which lead to war**.
I'd like to say the Barbary Wars and War of 1812 were completely justified as well. But I haven't done much research on them so I can't form a solid opinion as to whether it was 100% nessessary for our government fight them.
* The Southern states could have formed their own country(s) in 1787 without the need to fight a war had they not ratified the US Constitution.
** The terrorist attacks of 9/11 would most likely not have happened if our government had not persued an interventionist foreign policy in the Middle East over the last few decades which angered many Muslims and encouraged the growth of extremist groups.
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While the latter of those wars might have been justified in principle, they were still fought by unjust means - through taxation and conscription.
Pro Christo et Libertate integre!