Robb got suckered into the idea of splitting the north from the rest, to remain a friendly to any new king of King's Landing, but sacrifice no men to any future wars of the iron throne (after killing the Lannisters of course), however Stannis with his new god Rhollor and Melisandre telling him he must rule all the lands of Westeros meant he didn't like the idea of a King in the North. Even that could probably have worked itself out, he is the rightful heir after all (ignoring Dany because her existence is more or less unknown), but then you had Renly who was being a super annoying spoilt brat and somehow decided he would be King, he would not bend the knee to Stannis, nor Robb. Robb was on the verge of an aliance with Renly but due to Melisandres insistance Renly met a dark demise.
Moving on to a Robb, Stannis aliance, well Tywin has pretty much cut RObb off from forming any forward attack on King's Landing meaning there's not a chance of a commbined attack on King's Landing. This is all of course before you even consider the difficulties Robb has in book 2 with the north being picked off castle by castle by the ironborn.
THat's what I've taken from the books anyway, sorry for my ****poor grammar btw.