The royal family, the Mavelans, were mostly powerfully Mage-Gifted and unscrupulous. The throne often changed hands between brothers and cousins through both magical and mundane treachery. The Mavelans could see the magical energy of the ley-lines running into Lineas and pooling under the palace in Highjorune, and they wanted that power. The Lineas-Baires War was only the latest attempt to grab it.
When the war was eventually settled by a treaty negotiated by Valdemar, the Linean king's cousin Vedric, chose to pursue another course. As the treaty provided for each family to inherit the other's kingdom if one line died out without any help from the other, Vedric used a visit to his sister the treaty bride, Ylyna, to set a trap-spell in the Linean palace. Activating the spell remotely years later should have killed all the inhabitants, leaving no one to claim the Linean throne, and no traces that the Mavelans had been involved. Vedric did not count on his nephew Tashir surviving, nor did he count on Herald-Mage Vanyel investigating.
After Vedric's defeat, and the resulting deaths of the entire Mavelan family, Tashir inherited both kingdoms. However, having been Chosen by a Companion seemed to preclude him taking the throne. Vanyel suggested the Lineas-Baires Annexation to resolve the situation. The two kingdoms were combined and absorbed into Valdemar, becoming the March of Lineas-Baires, with Tashir as its Lord-Baron.