You have your new, fledgling empire established and need to expand it or be exterminated. In Crusader Kings 3 you need to have a Casus Belli available so you expand in a more “direct” manner than just playing nice or seducing queens.
In CK3 you gain a Casus Belli by fabricating a claim or by unlocking perks like Forced Vassalization and Ducal Conquest. There are innovations specific to a culture, like Chronicle Writing or Casus Belli, that provide new Casus Bellis, plus there are perks affecting the cost and speed of Casus Bellis, like Sanctioned Loopholes and Bellum Justum.
A Casus Belli is simply a reason to fight a war and it will have a specific war goal, like a territory. No matter how badly you defeat your enemy this is all you can gain if you fight a war to a successful conclusion. Other Rulers view a valid Casus Belli as a sufficient reason to declare war on someone.
CK3 Fabricating a Claim
One of the easiest and most ways to get a Casus Belli is to have your Bishop Fabricate a Claim. To start fabricating a claim go to your Council. On the left side of your Archbishop’s portrait there are three icons, the bottom one is for fabricating a claim. Click on it and you’ll be taken to the Fabricate Claim on County screen. You can then scan the map for territories you might want to stake a claim on. A tooltip will appear telling you the pros and cons for each territory as you hover over it.
For instance, you might gain an unpressed claim, but possible side effects could include things like a loss of vassal opinion, loss of Piety, or an upset target. It will also give you an idea of your monthly progress and the reason why. Those reasons might be due to your Bishop’s learning skill (usually a pro) or making a claim on a non-adjacent county (con).
Select the territory you want to fabricate a claim on and click it. You’ll be asked to confirm your selection, and after doing so, your Bishop will begin working on his new project. Beneath his portrait you will be told what county he is working on and how many months are left until the Casus Belli is ready. Once it’s available you can declare war on the Ruler of that territory after you pay the amount of gold required to acquire the Casus Belli.
CK3 Perks That Give A Casus Belli
There is a Casus Belli titled Forced Vassalization that allows you to force an Independent Ruler to become your vassal. You get this Casus Belli by unlocking the perk for it in the Diplomacy Lifestyle, in the Diplomat branch. Once you have enough perks unlocked the Forced Vassalage perk will become available for use when unlocked.
The Forced Vassalage perk allows you to force any Ruler with a lower rank who does not have more than three counties to become your vassal. Limits are increased by two each for the following innovations: Casus Belli, Chronicle Writing, Divine Right, and Rightful Ownership.
There is a perk prior to the Forced Vassalage perk in the Diplomat branch that gives you a new Casus Belli, and that is Ducal Conquest. Unlocking this perk gives you access to the Ducal Conquest Casus Belli. This Casus Belli allows you to seize “counties required to create Uncreated Title of Duchy rank.” To make it simple you need to own a county in the same uncreated Duchy as the county you want to annex. It saves you time and gold by not having to fabricate a claim, but it will cost you more in prestige.
There is another useful perk in the Diplomat branch of the Diplomacy Lifestyle when it comes to a Casus Belli, and that’s the Accomplished Forger perk. Although it doesn’t give you a new Casus Belli it does increase the Fabricate Claim speed by 75%. Once you’ve unlocked the “Forced Vassalage” perk it would be worth pursuing since it’s one of the next perks that’s available to be unlocked.
CK3 Innovations That Unlock Casus Belli
Innovations are another way to gain a Casus Belli, although it depends on the era you’re playing in and the culture you have as to which ones you will have. The best thing to do is check what innovations your culture has by clicking on the little candle icon at the bottom left part of the screen, close to your character’s portrait.
For instance, the Anglo-Saxon culture has access to the Casus Belli innovation which gives you access to the De Jure County Casus Belli while the Czech culture has access to Chronicle Writing which again allows you to use the De Jure Duchy Casus Belli. The difference between the two is that the Casus Belli innovation speeds up the average development of Anglo-Saxon counties by 0.1% while the Chronicle Writing innovation reduces the cost of prestige for using a Casus Belli by 10%, plus it increases the average development of Czech counties by 0.2%. Although they both give you access to the same Casus Belli there are differences between the culture innovations.
CK Perks Providing Other Casus Belli Benefits
There is a perk in the Learning Lifestyle, Scholar Branch, Sanctioned Loophole, that will allow you to use the Buy Claim interaction to buy unpressed claims for Piety. Among other things buying a Claim will give you a Casus Belli for declaring war to acquire a county. You can also get claims through a marriage.
There is one other perk, and while it also will not give you a new Casus Belli, it will reduce the cost of a Casus Belli by 50%, and that is the Bellum Justum perk. This perk is found in the Military Lifestyle, Strategist branch.
How To Declare War
Once you have a Casus Belli declaring war is relatively simple. The easiest way that I’ve found to declare war is the Current Situation menu. Opening it up will reveal all the characters you can declare war on. Clicking on the character’s name in the menu will bring up the Declare War screen. This screen will list all the valid Casus Bellis you have against this character (no Casus Belli, no war). The map will also highlight friendly and enemy territories and at the Declare War screen you can see who your allies are and who the enemy’s allies are.
Once you choose the Casus Belli you can see what will happen if you win the war. This part of the screen will also tell what their military strength is compared to yours and how much prestige it will cost you to declare war. If everything looks good click on the Declare War button. You’ll get a notification that you declared war on this character and the Raise All Armies button appears.
It will take several days for all your troops to arrive at the Rally Point. Once they’ve all assembled, you’re ready to march off to war! Enemy holdings close to you or the enemy’s capital are prime targets to go after, only engage an enemy army if you’re reasonably sure you can win the battle.
You can also right-click on a character’s portrait and declare war on him there. If the declare war option is grayed out it means you have no valid Casus Belli to declare war with for that character.