was dorne defeated?
It not true that Targaryens never married out of their own siblings. When Targaryens won westeros, Dorne was un-defeated and it came under their rule by the marriage of Daeron Targaryen to Myriah Martell. After that also few marriages were made between Targaryens and Martells and finally Dorne came under Westeros and thus the formation of 7 kingdoms.