As Queen of Aquitaine, I was pressing a weak claim war on the Duchy of Anjou on behalf of one of my vassals when he died, causing the war to be cancelled. The vassal's titles were inherited by his (12-year-old) son; as, of course, was the claim on Anjou because it had been pressed in war. The son is, naturally, still my vassal, and the Claimants page for Anjou shows him at the top of the list (which I assume means he's first in line).
However, I am now not able to Declare War over Anjou again, and the tooltip says it's because I don't have a valid Casus Belli. Why is this? I have a vassal with a weak claim on the Duchy. Is it because he's still under 16?
Best Answer
It's probably because he's under 16 (and would need a regent).
Unlike strong claims, which can be pressed at any time, weak claims can only be pressed against a character who is less fit to rule. Targets for weak claims include regencies, female rulers, and titles which are already being contested.
In the first two cases, you must also be "more fit" to rule. This means a female character cannot press a weak claim against a female and a child or incapable character cannot press a claim against a regency.
The Duke of Anjou may also have become ineligible as a target. The most likely reason he was eligible would have been regency, which may have ended by him dying/recovering/growing up.
Check that the duke is weak enough and will still be weak enough in 4 years. Alternatively, check how high your 12 year old is in the line of succession. You might be able to get him the title without a war if the right people have... unfortunate accidents.