It should then instantaneously turn into an invasion casus belli like someone said, or alternatively, a casus belli which deposes the murderer. Besides that, it's not actually stupid. There's way more stupid things people have fought wars over-- as close as the 19th century,the British declared war on the Qing Empire simply because the latter reportedly insulted their flag.In the Middle Ages for example,the Count of Tripoli declared war on the ERE and pillaged Cyprus simply because his family was insulted when Manuel Komnenos refused to marry his sister due to rumors which suggest his sister and by extension himself, might be products of his mother from an adulterous relationship.An actual rescue war could actually be seen in the form of what Richard the Lionheart did after Isaac Komnenos captured his wife and sister.Going to war over a prisoner seems a bit stupid, though. Not that you shouldn't be able to do it. As soon as the war would go badly for the one who holds the prisoners, they could just murder the prisoner, say he or she died in prison, and the CB would be invalid, so the war would end. (Also, if you capture the hold in which the person is hold prisoner, you should be able to free him or her right away).
Besides, plotting a prison break is way cooler.
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