So every NPC in the game lowers their opinion of me by 2? That seems unrealistic, 1 seems like enough...
The mechanic operates on the idea most NPCs will have a 0 or -1 opinion of you. So long as you're not waging wars of aggression or religious wars, NPCs will never declare war on you, more or less. (Unless you do some nasty backstabbing)
And besides, its not a war of aggression, its a boundary dispute.
A boundary dispute is when nation x claims territory A and then nation y claims that same territory. Nation X then gets a casus belli on nation y.
Ethiopia claimed parts of Sudan last turn, whereas you did not, meaning there is no dispute, only a violation of your SoI... but SoIs are not protected by game rules.
---
*obligatory ancient remix of Hell March here*
The Nubians, believing all of Sudan to be their rightful land, have declared a war of aggression against the Ethiopians. The Axumites have built up their forces and have prepared to weather the coming storm... 13 Nubian legions against Axum's 7. With enough luck, Axum can emerge victorious and repel the Nubians, as well as repay the intended mischief with interest.
By default, Nubia gets +0.6 to all its rolls, with 8% applied on top of that.
23-5. 6 Axumite armies remain.
10-6. 5 Axumite armies remain.
9-23.
15-3. 4 Axumite armies remain.
10-24. The attacking army was destroyed.
14-3. 3 Axumite armies remain.
4-19. The attacking army was destroyed.
25-3. 2 Axumite armies remain.
1-13.
23-3. 1 Axumite force remains.
25. 0 Axumite forces remain.
Nubia's 2 surplus forces overrun two Ethiopian provinces. Despite fears that 13 legions would bring bad luck, Nubia has done well... having conquered 9 provinces!
However, Nubia did lose 2 of its 13 armies... it also now defenseless, due to all its forces being away on campaign! The people are not happy about this war either. Nationalist as they are, the Axumites didn't really do anything worth punishing besides trying to improve opportunities for their people, same as every other culture.
Making matters worse, besides the increased instability of the vast Nubian empire, the international community is unlikely to smile upon this behavior.