For a complete list of Casus Bellis and explanation, go here Types of Casus Belli
While some of these CBs you'll have merely due to your position, like the independence CB and the law change CBs, many of these can be gained on purpose.
First, the easiest type of CB to gain are claims. There are four primary ways to gain claims:
- Forge the claim using your Chancellor. This doesn't work for anything above the ducal level (skill of 15 needed to forge a ducal claim) (or for anthing below the county level, like a barony)
- Marry someone who has an inheritable claim. Your children will have the claim and you can then push it
- Marry someone in your dynasty to someone who has an inheritable claim. Their children will have the claim, and you'll be able to push it
- Invite someone with a claim to your realm. You'll then be able to push it
Second, de jure ducal claims are also relatively easy to gain. If you can create a duchy that overlaps another realm's territory, this will gain you a de jure claim, and the same applies if you usurp it. In addition, after pushing a claim for a duchy, you'll often have de jure claims.
Third, dispute succession CBs can, like general claims, be gained by inviting someone with that CB to your realm.
Fourth, excommunication CBs are easy to get as long as the pope likes you, but these won't help you grow your realm. You can only depose the ruler. They're good for gaining some prestige, though.
Fifth, you can attack someone of a different religion to claim a Duchy. If you are Catholic, you can attack Pagans, like the Balts and Finns. You can get religious war CBs on essentially the entire world if you convert to a Christian heresy. The easiest way to get heresy to appear so that you can convert is appointing an antipope, as this lowers the moral authority of the church. The lower the moral authority of the church, the more likely heresy is to spread. Then upon the first chance you get, you should change to whatever heresy arises. You'll now have CBs upon everyone except other realms following the same heresy.
So while the drawbacks of heresy are severe (-30 opinion with all non-heretics, and increased rebel risk), the advantages when it comes to expansion are huge. That said, adopt this position with caution; while this approach means gaining a casus belli on everyone, it also means everyone gets a casus belli on you! Which means that unless you are the largest fish in the pond, things are likely to get very ugly fast, and even the almighty Holy Roman Empire is vulnerable to dogpiles.
Using Your CBs
Pushing your own claims and similar is rather straight forward. When pushing someone else's claim however you have to be careful, or they'll end up as an independent realm. They'll only be your vassal if they are already your vassal, they're in your dynasty, or the claim you pushed was for an area that is de jure part of your realm. If the title they took is at the same level as yours or above they'll be independent even if any of the above conditions apply, unless you have a liege above you where the conditions still apply.
The easiest way to ensure that they'll be part of your realm after pushing their claim is granting them a county in your realm first. The only way they can become independent then as a result of the claim pushed, is if the title they gain is at the same tier as yours, or higher.
Casus belli and claims are easy to gain as long as you know what you're doing. Following these tips you should be able to expand easily.
- All wars except tribal invasions incur 10 years of truce
- Marry claimants with inheritable claims to gain more claims
- Marry your dynasty members to characters with inheritable claims
- Invite males with inheritable claims and grant them a county
- Invite pretenders and grant them a county
- Make the pope love you and you can farm prestige off of excommunication
- Go heretic and you'll have CBs on everyone
- Create or usurp duchies that overlap the territory of other realms
Contributed by Meneth