The Manifest Path/War
While most resources should be put toward defeating non-human forces inimical to humanity, there are on occasion reasons for war to be fought between humans.
In essence, when asking if a war is just, the answer must be: Only if it improves humanity as a whole.
How could a war improve humanity as a whole? Several ways.
If resources are not being utilized properly, then a war to see that they are utilized better by another could be justified. If a realm claims lands it does not utilize while others have the ability to take and build those regions up, then a war to take the claims would be justified.
Obviously, those humans which have sold out the rest of us to inimical non-humans must be cleansed, and war with them is always justified.
These are merely a few examples, but always remember the key question: Will this improve humanity?