But it wasn't a war between sovereign nations. It was a rebellion or, at best, a civil war.
I think that, as Lord Palmerston noted, it is hard to say that the Confederacy didn't fulfill all the practical requirements of a sovereign state. They had a working government with a functioning executive and legislature; they had an army and a navy.
But even if that is true, that doesn't mean that it wasn't a civil war. The connections between the two combatants, at every level, were close in a way that is not characteristic of any other war between nations. Brothers fought brothers.
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