Just Read RW (Spoilers)

[AegonTargaryen]

That's true, by that point the situation was bad. But even then, had Robb been able to bring the Freys back into the fold, I'd argue there would have been a stalemate more than a Lannister victory. Robb would have retaken the North and still held the Trident. At this point, assuming Riverrun falls (huge assumption), Robb and company hide behind Moat Cailin or at Greywater with the Reeds, and the war drags on indefinitely.

Spoiler from end of aSoS below

let's also assume no butterfly effect and the Red Wedding doesn't affect events in King's Landing. With Tywin and Joffrey both dead and Tyrion gone, the Lannisters are also on the brink. Cersei and Tommen don't have the wherewithal to end the Robb's rebellion. Point being, If Frey and Bolton don't betray the Starks, anything could have happened.