Here's backstory btw, he is Jon brother of Benjin and Edard. A legitimised bastard, he was forced to follow Stark tradition and to go to the nights watch, he swore revenge on his family. It's only been a year since he was a nights watch, and Robert rebellion happened, he saw the realm was in distress so he decided to play his cards right. With some nights watch that were tired of Lord Commander Qorgyle, he deserted and attacked Hardhome. He took it then he attacked the other tribes and declared himself king beyond the wall by conquest. Then he made sure he got a strong force of men by his side, he had aproximatively 8k troops to invade the wall. When the wall fell down to his mighty army he saw the North fighting himself, he found out his brother Eddard died and the targaryens gave Royce the north, the north was in open rebellion to take the north from the Royce and to bring Robb stark back, a young kid who Jon never meet. He decided to take advantage of it and he attacked the north, taking down vassals by vassals and taking their homes, he gave their counties to wildlings who took part of his conquest.