The only good (and moral) reason there ever has been to fight a war was the legitimate defence of one's own homeland against a direct attack by a foreign entity... and the legitimate resistance against invasion of one's homeland and the occupation of it by foreign armies.

Any other excuse is just that: an excuse.

There is one other kind of war, though...an internal war within a nation. A civil war. That's a particulary nasty situation, and it can get damnably complicated. It is often further complicated by the involvement of foreign powers in various ways...as they may see some advantage in involving themselves. If so, it gets even worse.