In Crusader Kings 3 you cannot just declare war on anyone for no reason at all. You need to have a Casus Belli, which means cause for war. Fortunately, there are many different types of casus belli that you can get during your playthrough and a lot of these will have to do with character claims, your religion or government type as well as de jure claims via innovations and research.
I won’t cover every single possible casus belli but here is a page where you can take a look at all of them, including some custom ones that come from undertaking decisions, getting lifestyle perks etc.
I’ll briefly explain the most important ones below.
Character Claims
Early in the game, you will most often go to war with other rulers based on the claims that you personally hold, or someone else who is a part of your court. Holy wars are common as well, more on that below.
Claims can be passed down from parents to children (up to a point) and after that, they disappear.
Other characters with claims can show up in your court and if you recruit them (have them join your court permanently), you will be able to press those claims and go to war for them. Once successful, that character becomes a vassal of yours.
Just be careful not to go to war for someone’s claims if the end result is them becoming a ruler of the same rank as you (they will become independent). This means that if you’re a duke, you never want to go to war for someone else’s claim on a duchy. Making them a count is fine (they become a vassal).
And also be careful when recruiting women with claims because often you won’t be able to go to war for them if the current ruler of that land is not a woman, or the land’s laws do not allow women to inherit the throne by default. This will obviously be bypassed when there’s no male heirs, which happens often.
However, if a woman with a useful claim shows up, always check that land’s current situation. If there’s another woman on the throne, you can go to war and install your courtier instead.
This is important because you might make the mistake of paying 200+ gold to recruit a woman with a kingdom claim, only to find out you can’t go to war for it any time soon. And by the time that you may be able to, that woman might die of old age. 🙂
Claims can be implicit, pressed and unpressed. Consult the game’s tooltips for a quick explanation about each of these. They’re not hard to understand at all.
You will often fabricate unpressed claims to get specific lands for yourself early on. This is a task your realm priest can perform.
Religion & Holy Wars
In order to declare a holy war on another ruler, your religions need to consider each other hostile or evil. This means that you cannot declare holy wars on rulers that belong to a religion that isn’t sufficiently different from yours.
In addition, once a holy war is declared, other rulers of that religion can join in as defenders so be careful when trying to invade kingdoms (you can do this once per lifetime).
Holy wars for duchies and counties do not have this limit. You can keep declaring them as often as you want. Other rulers can still join but usually won’t bother with smaller pieces of land.
You spend piety to declare holy wars. The size of the war’s target (title rank) determines how much piety is needed.
When someone else declares a holy war on you, if you are about to lose, the game will prompt you to convert to the attacker’s religion and thus invalidate the war automatically.
This should be used as a last resort only, since converting to a new religion destabilizes your realm and other characters opinions of you. Plus, other rulers might now be able to declare holy wars on you as well.
But at least you won’t have to worry about a large empire attacking you again on the basis of a holy war.
Conquest Casus Belli
This type of casus belli only allows rulers to invade their neighbors that are bordering their own lands. Tribal governments have access to this casus belli, as well as religions with the appropriate tenet.
You can create a custom religion and add this tenet to enable yourself (even while playing feudal) to go on conquests against neighboring rulers in addition to combining this with holy wars.
This cause of war requires prestige, while holy wars require piety. It’s a good idea to keep alternating between the two for maximum results. It’s one of the fastest ways to expand quickly.
Innovations & Research
Obtaining de jure casus belli simply means that you’ll get a cause for war on all lands that are rightfully supposed to be a part of your highest ranking title. For example, if you are a duke but do not hold all counties inside a duchy, you’ll now have a rightful casus belli for getting those other counties.
Similarly, in the next era of innovations, you can research de jure duchy casus belli innovation and claim other duchies that are a part of your kingdom (unless vassals already hold those).
And lastly, once you finish researching the last era innovation, this will give you the overall de jure casus belli which means that any and all lands that are a part of your highest ranking title (ex, empire not holding all kingdoms) will automatically grant you a casus belli if there’s need for one.