When is it appropriate for another nation to use military force in another nation's affairs?

IMHO, it's a fine line.. The US pretty much ignored Germany's advancement upon Europe in the 1930's, only to have Japan disobey German authority and attack the US before Germany had Europe under it's control..if Japan had waited like Germany had planned, the combined forces of Germany and Japan probably would have defeated the US, or at least invaded the homeland.. things might have been a lot different if Japan hadn't been so arrogant.. that being said, where do you draw the line? If aggression is ignored, and allowed to gain momentum, instead of "nipping it in the bud", who's to say what might result... no one really knows for sure...all we can do is guess and speculate..