That's the problem people have grappled with for centuries, all over the world. One side starts/provokes the war, so obviously they're in the wrong. But what of the other side? Are they unjust merely by fighting? If not, should they content themselves with merely repelling the invasion? should they carry the fight to the enemy? what constitutes what in Arabic is called "i'tidaa'"--to exceed what is necessary and just? And as you note, Shane, what is "just" in this case?
We've had to grapple with the problem in the Near East since at least the Bronze Age, and we've yet to find a (near)perfect answer. Even religious and legal scholars in the middle ages couldn't agree: the Shafi'is traditionally believed it was just to fight a country simply because it is ruled by an infidel/unjust regime (an attitude not too different from neo-cons in reasoning. Just replace Sunni Islam with Democracy).
Others believed there had to be some serious violation of what passed in those days for international law (killing an emissary, piracy/privateering, invading your soil, etc). While this is more noble in theory, it can easily be seized by a clever sultan to justify aggrandizement. For example Suleiman I used the humiliation of an emissary are an excuse to attack Hungary in the 16th century. Perfectly legal, but what Suleiman I did as a result wasn't necessary
, and in the long run might have made the Ottomans more vulnerable. This was by dragging in the Hapsburgs into the war (they had a claimant for the Hungarian throne, whereas the Ottomans, after killing the Hungarian King at Mohacs, wanted another candidate: wisdom would have recommended leaving the Hungarians to fuck themselves over--they were already doing a good job of that prior to Mohacs; the Austrians would have taken a long time to sort out the mess--Austria was historically pretty bad at this sort of thing...).
The only conclusion I can come to myself on the subject is that there is no such thing as a truly just war
; only necessary
ones*. And those should involve defense of home and hearth. This may or may not entail taking the fight to the enemy, but it's hard to tell (if not impossible) when you should or shouldn't.
* to not resist is either to allow occupation: sometimes that may be acceptable, but sometimes it's not (e.g. when dealing with ISIL).