If the Agrots invade the Naratlans in the the Naratlan's current home territory of Farcen, then clearly the evil Agrots are agressors and all right-thinking peoples should strive against their villany.
But if 10 years go by and Valrani invade the now-comfortably-settled Agrots, should anyone care? If 100 years went by? 1000? At what point do the Agrots "own" Farcen?
And would the rules be different if it were the Naratlans returning to retake their former homeland? Would we give them a greater statute of limitations?
Why do we teach our children to be honest and that laws are important, but then teach them that reporting dishonest and illegal behavior (“no one likes a stool pigeon”) isn’t admirable?