Now that is a very good question. I guess there could be lot of ways to find justification for a war. For starters or on academic level consider the two criteria listed below (right to war on right and international human rights law on left) which could be a good starting point but not comprehensive by any means in my opinion.
Now with this said the key would be 'just cause' and that is not absolute in my opinion but you could use a widely accepted definition of just cause that is agreed upon by people from all spectra.
Its very late and I am not even sure what I am talking about ;)