Before you go to war in Crusader Kings III you want to make sure you have the manpower and money to back yourself up because at any point in time your enemy could call upon its allies and become a force to be reckoned with.
To begin a conflict you will need a Casus Belli, this is an allegedly justified reason to go to war. Once you’re ready to start a war, you need to make a formal declaration by right-clicking on the enemy and selecting Declare War.
Here is a list of all the types of wars you can initiate in CK3:
Fabricating A Claim (Easiest)
The most popular way to go to war is by Fabricating a Claim via your Bishop, Archbishop, Allamah, your High Shaman, etc. This is generally the easiest way to go because it does not have many requirements other than a small bribe usually by your head of faith and a little bit of time to do so.
Holy Wars Require Faith
Holy Wars will require a certain Level of Devotion to your faith and there are five levels. When you want to go on a Holy War, it will require more from you depending on the size of the area you want to seize.
For example, if you want to go on a Holy War for a County, then you will need a level 1/5 faith because it is the smallest amount of land you can capture. Duchies require you to be “Faithful” which is a religious faith level 2/5. However, if you want to go on a religious crusade and take a Holy War for a Kingdom, it will require you to be a Paragon of Virtue (4/5).
You may only attempt one Holy War against an entire Kingdom once in your lifetime, also known as a Great Holy War, so make a wise decision! Your faith must follow certain doctrines to go on a Great Holy War. Holy Wars are never an act of Tyranny and thus you will not suffer that cost, but you still must pay for raising troops and your Casus Belli cost as usual.
Subjugation is another way to go about war but can only be done once per lifetime. Essentially, you are annexing another ruler and vassalizing them if you win. Subjugation can be initiated for any size of land you seek.
Seizing a county can happen if you have the tribal-era innovation “Casus Belli,” a duchy if you have the Early Medieval Era innovation “Chronicle Writing” and you can seize an entire title if you have the Late Medieval Era innovation “Rightful Ownership”.