To claim a duchy from somebody else through war (when you don’t have a casus belli that lets you claim it directly): Find out who* has a claim on it by opening the duchy information screen and clicking on “Claimants” to see who has a claim on that kingdom. Invite one of the male claimants over to your court.
How do you press courtiers claim ck3?
If you don’t have the titles to spare or if you just aren’t in the mood to fight other people’s wars, then the easiest way to gain a claim in Crusader Kings III is through your religious advisor on your council. They will be able to take the action to press a claim on a territory.
When do you need a claim on a duchie?
If they are not of your dynasty or your vassal, the duchy will not join your realm after the war (although the claimant still gets the title). If the current duke is adult male, they need a strong claim, otherwise a weak claim works as well.
How can I usurp the title of a duchie?
If none of the above options work, then take one of the counties in a larger duchy that will allow you to usurp the duchy title (you need over 1/2 of the holdings in the duchy). Also, if you are loosing claims to duchies, this could be for a number of reasons.
How can I make a claim on a Duke?
If the current duke is adult male, they need a strong claim, otherwise a weak claim works as well. You can get a list of claimants by clicking on the coat of arms of the duchy you want (or a county in that duchy and then navigating to the duchy by clicking on the CoAs at the top of that screen), then clicking on the claimants button.
Why are non-heritable claims on duchies available as casus belli?
If the claims you had last time were fabricated by the chancellor, note that they are not heritable. However, non-heritable claims become heritable if you press them in a war (even if you loose the war). (As an aside, once you are an emperor you can vassalise the pope. This is great.) This doesn’t answer the question.
