M
Mjölnir
Guest
He's also Warden of the North, which will be the title actually used in practice.He's not the King in the North, he's the Lord Paramount of the North since the Starks are now an exiled house.
He went south to get Ned as roach pointed out. His biggest mistake was declaring independence. He should have declared for Stannis or Renly, told them to agree/fight amongst themselves but would swear fealty to whoever won. Coordinating with either one would have given them the advantage over the Lannisters. His presence in the Riverlands would have forced Tywin to divide his forces in the West and near King's Landing. The Tyrells only play for the winning side so it would have been likely they would join a Stark/Tully/Baratheon alliance. Plus, if Renly got killed, he could have married Margaery.
Not to mention the real war will be against the White Walkers. The Northmen would have be critical for that since they are the soldiers most capable of fighting in winter.
As for Robb's actions, he tried to reach out to Renly through Catelyn. Had Renly not been assassinated there could quite possibly have been an alliance between him and Robb. Renly was willing to make one, and would even let Robb call himself King in the North as long as he swore fealty. That was pretty generous considering that Renly didn't actually need any more allies, had sorcery not been involved.