What if another country is exterminating its populace systematically, would it not be just to attack that country and stop it?
I am not aware such ever had been done only because of that reason, but it is cosntantly used as reason more as excuse, as it makes the population of the helping/invading army feel much better about themselves, that is exactly for what that line of reasoning is been used.
To answer your question, in the case this happens, and it would "not" be about economical reasons, then it still "isn't just".
War is just only when you have to defend yourself, is a premise, a doctrin, a law, an unmovable object where when a lever applied would lift the wolrd from its hinges befor it becomes untrue.
Still i would feel morally obligated to intervene, as you make yourself accesory to the crime if you don't help.
Lets see it as an exception to the rule. Are there other exceptions, maybe, but there are not other ideals which would allow anyone to wage war other then to defend himself. If people would accept that, who would ever gofor an attack in the first place ...
only criminals So anyone who attacks soemone is a criminal. If we define that in all of our constitutions or a treaty between/within the UN members, with a defined set of consequences and punishments for the criminals, ...
edit: several treaties concerning an international court, the USA didnt sign, instead a senator said in the parlament something like "if at anytime an US soldier would end up in front of that court, the US would invade denhaag if needed to set him free."
Perhaps we EU pussies need still a bit more trust from the USA to make this work, till then we just participate with the UN, situated in the USA.