I think Robb declaring himself King in the North was one of his biggest mistakes. He let the Greatjon and the rest of his bannerman gas him up. When he did that any possibilty of an alliance with any great house vanished. None of the people who laid a claim to the Iron Throne would have accepted being king of only half a kingdom. Even bytch ass Renly warned Catelyn about that.
Another great mistake was sending Roose Bolton away, and not just because it would have prevented the Red Wedding. If Roose was there to throw cold water on the KITN idea and advise Robb adopt a more...conciliatory position, Robb could have allied himself with Stannis and the war would have been won.
He sent Roose away because Roose scared him.
Robb was a great warrior but his political acumen was lacking. He was Ned's son indeed...