The nation-state is a bourgeois construction. Marx does say that the workers are without a nation (Can't be bothered to find the exact quote at my fingertips). As such the destruction of the Capitalist Superstructure necessitates the destruction of the very concept of a nation-state.
I have never read Das Kapital (Though I fully intend to once I can find a print version). I have, however, read quite a few of the more obscure writings of Marx, at least in part.
EDIT: Though I do believe we have strayed somewhat from the original question... my mistake.
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