I can see that, too. having done a lot of genealogy and having read and transcribed my Civil War ancestor's letters, I would guess that the average Northerner had no animosity against the Southerners. He and most of the men he fought with were fighting to preserve the Union. Since the north prevailed in the end, the Union was preserved. Since the Union was "reinstated", from that point of view there would have been no need for naturalization. I think that in their minds, since the South lost the war, they were still part of the Union and therefore still US citizens. Note I say from the idividual's point of view, not the official government stance. And again, since most of the men in the north (at least in 1862-63) were fighting to preserve the Union, with the Union preserved, we were all once again in the same fold as we were supposed to be. (Just trying to put words in my ancestor's mouth. Geuss I'll find out if I;m right in eternity.)