When is war applicable?

I’ve been a libertarian for most my life. As an economics student it makes the most sense to me. However, as I am as antiwar as the other libertarian, when do we need to go to war?

I only ask because when we say we stand for liberty and justice but certain countries deprive others of those liberties, when is it appropriate to step in. Like when certain countries limit women’s rights or sell children into slavery or murder gays or lesbians, at what point do we say enough is enough?

Was WW2 justified? I don’t want to police the world, but at some point you can’t let atrocities take place, just wanted another perspective.