I guess there is no logic. Yesterday I was playing my campaign and my faction (Khuzaits) was waging a war against Aserai. It was a long war, about 2 years, but when the Southern Empire declared war on us, Khuzaits and Aserai made peace. "Wow, some logic", i thought, and hurried at the SE's territory, but as soon as the sun rose above Calradia, guess what... Aserai declared war on Khuzaits again! In a day! What logic could be behind this?