I am enjoying the WE playthrough at the moment. Tried to join them early on, but poor Garios got imprisoned by the Battanians. Afterwards it was back and forth, with the filthy barbarians getting the upper hand most of the time. The turning point was when I accidentally stumbled upon Caladog returning home after a huge battle (had many prisoners on him). I defeated his weakened army, executed three Battanian lords and at the same time - freed Garios and couple of our lords from captivity. Now Garios is gathering an army and I think we should be finally taking one of their settlements. By the way, Battania seemed to cope quite well with waging war against WE and Vlandia for the last 6 months.
TLDR - it's random. Become a vassal and help out. The Empire army is the easiest to master and there aren't many weak points.