Technically never, because a State military is illegitimate. But if you’re asking when can you legitimately go to war, the short answer is when you are legitimately threatened. But for a true answer, many others have written on this. Here’s a few samples:
Can a Principled Libertarian Go to War?
The Justice and Prudence of War: Toward A Libertarian Analysis
Is Humanitarian War the Exception?
For one thing, you personally can do whatever you want. Just don’t force me to do anything…including pay for your choices. But for another, you might want to consider the real-world implications of humanitarian war.