A less stated (but well understood) component of this story is that Clemens grew up believing in his southern slave culture, fought (if joining up is fighting) in defence of the southern side, but converted later in life, not the least because he married into an abolitionist family.

A quote I like: "Samuel Clemens later observed, "Civilization began when slavery was abolished."

Many in the south remained unreconstructed racists and opposed to Grant. Clemens didn't.