Tsura Sobasa's feed

Did I miss something or was there no mention of why the Argan empire was invaded and why the victors needed the princess to die until ch 17 at least? If it wasn't because they hated magicians and needed to make an example of her it would make more sense to marry the princess of the fallen empire off, which is what is usually done in stories like these. Seems like bad storytelling to me if that information wasn't lost in translation.