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What kind of identity would the North develop? I'm thinking a successful Confederate secession would be something that promotes bitterness and desire for revenge rather than "we're the true heirs of the Revolution and good riddance." It would be massive blow to the image of an ever-growing, ever-improving free nation.
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Why would losing the South stop growth?
Sure you just took a couple of steps back, but you still have plenty of space to grow to the West. And if the Confederates never get to the West Coast because the USA sucked up all the available land, wouldn't that be a shame...