In Season 1 episode 2, Princess Rhaenys & Lord Corlys Velaryon {the sea snake} offered the hand of their daughter Lady Laena in wedding to King Viserys I Targaryen.
They proposed that he should take their daughter as his new queen because this marriage would strengthen the royal line, produce more heirs, demonstrate a strong alliance between the Valerian families and unite the realm against enemies thereby protecting the throne which is perceived to be weak and vulnerable to attack on several fronts.
