Western campaign.

Robb’s whole Westerland campaign seems extremely improbable. A hidden pass large enough for a army and to drive Cattle through, yet somehow hidden from the natives of the region. Crossing over a massive distance without the army being alerted by Ravens from any of the holdfasts or riders fleeing ahead. Then capturing a number of castles which seem to fall quite easily. The whole plan to beat Tywin also rings false. Tywin has at least 2 times Robb’s numbers at minimum. Without a battle of the Fords and with Robb’s own losses assaulting and fighting, it could be 3 to 4 times his number. Then his plan to outmanoeuvre Tywin. Except Tywin is on his home turf, has untaken castles aiding him and the remains at Oxcross. And cavalry don’t really March faster on a strategic level then infantry. And that Tywin likely has equal numbers of cavalry anyway. And his only escape is that narrow pass with the Golden Tooth untaken. Sure it lures Tywin from King’s a Landing, but Stannis is a enemy as well. Helping him win by getting the North’s cavalry destroyed is extremely foolish.

I strongly disagree. 

1. The hidden pass thing is actually something of a historical cliche. It happened at Thermopylae repeatedly, it happened with Alexander, etc.
2. Castles falling without being able to alert others we’ve seen happen repeatedly in other contexts (Tywin’s march through the Riverlands, Theon’s capture of Winterfell, etc.). An unexpected attack against undermanned garrisons because the main forces of those Houses are off fighting with Tywin, post archers to shoot down ravens, and you’re good to go. 
3. Robb’s plan for the Westerlands makes a good deal of sense. Robb has repeatedly outfought larger armies, and as long as he can maintain local superiority and establish multipliers on his side, there’s no reason why he can’t win. Also, there are no remains at Oxcross, that army was wiped out. 
4. If Stannis takes King’s Landing, then Tywin loses the war politically in an instant, because he has no king to fight for, and the man on the Iron Throne will have him declared a rebel, outlaw, and traitor. This knocks out Robb’s main antagonist, and he’s willing to make a deal with Stannis once Tywin’s out of the picture.