So I've been playing a lot of Crusader Kings II because it's a fun game. One thing I've noted(and also angered myself over) is the Catholic's lack of Casus Belli, or cause for war. Naturally you wage a war because you want land which in turn gives you a title and whatnot. Unless you want to go to Holy War against the Muslim or Pagan rulers(which is the equivalent of sticking your privates in a wasps nest really) you need a valid cause. One of these is of course that you forge a claim to whatever area you want. Another is a thing called a "De Juer" claim. Basically this means you have a Duchy that traditionally consisted of Barony A, B, C and D. You hold the title of the Duchy, but Barony D has somehow ended up under a different ruler, so Baron D is not your vassal. This means you can claim that "This Barony is actually a part of my Duchy!" and you can call your valiant men to arms and go to war against the misfits that are there.

How realistic was this and was it often used? Was there other valid Casus Belli in European Middle Ages? Could you just declare war on someone because you didn't like them? What were the repercussions if you declared an "unlawful" war(i.e. no valid Casus Belli)?

Will clarify if there are any questions.