One of the oddest things that happened in a campaign was when i was plying as the Getai, it was about 260 BC and suddenly it was annoinced the arverni were at war with the Seleucids. I thought it was just one of those strange cases of the AI declaring war for no apparent reason, until a few years later one of my diplomats came across a full Arverni stack in Ancyra! I suppose an Arverni diplomat must have bribed the garrison. Certainly was bizarre though.