He had an heir, according to tradition, its just that a significant portion of the royalty didn't like her, and they all technically had a claim, its just tradition had developed such that the ruler of Cyre was the heir, but some people really didnt like her. Then the war devolved into I hate you cause you blew up my shipment/castle/brother, and I hate you back cause you blew up my allies shipment/province/hat.