Complex sentence: Her father arranged the marriage, after his attempt to betroth her to Rhaegar Targaryen as she wanted failed and Targaryen dynasty lost the war, initiating himself as a political counselor for the newly crowned King Robert.
Split into simpler sentences: Her father arranged the marriage after his attempt to betroth her to Rhaegar Targaryen, as she wanted, failed. The Targaryen dynasty lost the war, and her father schemed his way into the role of a political counselor for the newly crowned King Robert.