It'll come to no surprise to those who've played with me on Dwilight that I'm a big fan of rival duchies slugging it out within the same realm. But there are already plenty of ways to do that without the use of actual intra-realm warfare. Rival dukes have a wide range of options available to !@#$ with each other. Snatching knights away, swaying lords to change allegiance, hoarding food and gold, undermining their authority and influence, using personal armies to take all the glory away from others, good old fashioned slander campaigns, et cetera. These are all things you can do that don't require anyone seceding. The kicker is that if you push things far enough, secession can be the result... but that just makes things more interesting.
