Pact of Ice and Fire
At the start of the civil war, Prince Jacaerys Velaryon flew to Winterfell to gain House Stark and the North for the cause of his mother, Queen Rhaenyra Targaryen. According to Mushroom, Prince Jacaerys secretly married Lord Cregan Stark's bastard sister, Sara Snow, which led to the pact. It was agreed that the firstborn daughter of Prince Jacaerys would marry Lord Cregan's son and heir, Rickon.
Lord Roderick Dustin led the Winter Wolves to join the blacks, while House Manderly also contributed knights to Rhaenyra's side. Lord Cregan himself led a host to King's Landing near the end of the war, but they arrived after the death of King Aegon II Targaryen.
Cregan briefly governed in the capital as Hand of the King during the Hour of the Wolf. Although a royal princess never did marry into the Starks, Cregan gained many rewards for his support of King Aegon III Targaryen.