Justifiable from their perspective. Most border disputes would fall under this, at various points in time both sides have controlled an area and believe that it is rightfully theirs.
"Justifiable from their perspective" is trivial. Given the wealth of examples, which GGP can be assumed to know about, that's not what they were talking about.
It's the only situation where a build up of arms before war is understandable. Massing arms out of defense is basically indistinguishable from preparing an attack.
