“Physically speaking, we can not separate. We can not remove our respective sections from each other nor build an impassable wall between them. A husband and wife may be divorced and go out of the presence and beyond the reach of each other, but the different parts of our country can not do this. They can not but remain face to face, and intercourse, either amicable or hostile, must continue between them, Is it possible, then, to make that intercourse more advantageous or more satisfactory after separation than before? Can aliens make treaties easier than friends can make laws? Can treaties be more faithfully enforced between aliens than laws can among friends? Suppose you go to war, you can not fight always; and when, after much loss on both sides and no gain on either, you cease fighting, the identical old questions, as to terms of intercourse, are again upon you.”

1860s, First Inaugural Address (1861)
Context: One section of our country believes slavery is right and ought to be extended, while the other believes it is wrong and ought not to be extended. This is the only substantial dispute. The fugitive-slave clause of the Constitution and the law for the suppression of the foreign slave trade are each as well enforced, perhaps, as any law can ever be in a community where the moral sense of the people imperfectly supports the law itself. The great body of the people abide by the dry legal obligation in both cases, and a few break over in each. This I think, can not be perfectly cured, and it would be worse in both cases after the separation of the sections than before. The foreign slave trade, now imperfectly suppressed, would be ultimately revived without restriction in one section, while fugitive slaves, now only partially surrendered, would not be surrendered at all by the other. Physically speaking, we can not separate. We can not remove our respective sections from each other nor build an impassable wall between them. A husband and wife may be divorced and go out of the presence and beyond the reach of each other, but the different parts of our country can not do this. They can not but remain face to face, and intercourse, either amicable or hostile, must continue between them, Is it possible, then, to make that intercourse more advantageous or more satisfactory after separation than before? Can aliens make treaties easier than friends can make laws? Can treaties be more faithfully enforced between aliens than laws can among friends? Suppose you go to war, you can not fight always; and when, after much loss on both sides and no gain on either, you cease fighting, the identical old questions, as to terms of intercourse, are again upon you.