I think your question is asking whether starting a war is ever justified. It is clear, to me at least, that a defensive war is justifiable. It would be a strange world if countries could just start wars knowing that there would be no response.
So is it ever justifiable to initiate a war? I think it could be. For instance, if you know that another country is planning a war against you, not just thinking, but actually planning and gathering the weapons they will need at your border, a first strike might be appropriate. Of course, this all hinges on "knowing." The world "knew," for instance, that Iraq had weapons of mass destruction, but there were never found. (This doesn't mean the world was wrong, just that, by the time we got there, the weapons were gone.)
I think it also might be moral to "join" a war, even if "we" are not attacked. This might be the case, for example, to try to stop mass genocide in another country.
All this is very hazy as it happens; it's much clearer from 100 years away ....