Robb lost his purpose and that's why he lost the War.

[Bernie Mac]

7 minutes ago, Targaryen Restoration said:

They were expressing doubts whether the war could be won.  They did not switch sides until Robb betrayed them and Tywin took advantage of it.  I wouldn't claim no influence from Roose.  Perhaps he was whispering in Walder's ear but the Freys did not leave Robb until after he refused to marry a daughter of the house.  They left the battle field at the news.  It was not subtle when they refused to fight for Robb.

 The Freys, after learning that Stannis on the Blackwater had been defeated and Tywin had gained the armies of the Reach and most of the Stormlands, knew the war was over. They were going to give Robb the opportunity to sue for peace and stand by his side in doing so, but if he was to ignore common sense they would not have fought on for him. The war, at least the war in the South, was pretty much over. Robb, from their vantage point, simply could not win.

Keep in mind that Robb's own mother was telling him to sue for peace. Him refusing to do so in his position is stupidity or selfishness of the highest order. The Freys would have soon jumped ship, I'd bet they would not be the only Riverland House to do so.