Kajani wrote:On that point I disagree a bit. I think the rapid sucess of the Ironmen shows that the North had already sent a huge part of his men to the south. Several lords had not sent too much, but that the North was so much weakened that the mad Greyjoy thought it might be a easy prey says a lot (although this guy was never very wise when it comes to calculate the chances...

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Rapid success?
The Ironborn took three places - Deepwood Motte (by sea - their specialty), Moat Cailin (by coming up the Saltspear - and not up the causeway against which the ruin is notedly defensible), and after a feint on Torrhen's Square, Theon took Winterfell by trickery.
And notedly, as soon as anybody in the north decided to take one of those back, it happens easily.
Yes, the North sent quite a few men south, and so by definition their forces were depleted, but the issue is certainly more one of distance to cover in mustering and a general undercurrent of opportunism for many supposedly loyal lords and nobles than any particular weakness of arms.