...a just war exists when a people tries to ward off the threat of coercive domination by another people, or to overthrow an already-existing domination. A war is unjust, on the other hand, when a people try to impose domination on another people, or try to retain an already existing coercive rule over them....
There have been only two wars in American history that were, in my view, assuredly and unquestionably proper and just; not only that, the opposing side waged a war that was clearly and notably unjust. Why? Because we did not have to question whether a threat against our liberty and property was clear or present; in both of these wars, Americans were trying to rid themselves of an unwanted domination by another people. And in both cases, the other side ferociously tried to maintain their coercive rule over Americans. In each case, one side – "our side" if you will – was notably just, the other side – "their side" – unjust.
I share Mr. Rothbard's view of just war. If you are curious which two wars he refers to, please take time to read his article.