The question about Twain's Civil War experience has already been
ably answered.

But it brings up a question that has interested me for awhile.  Sometime
within the next few months, an essay of mine on Civil War influences
on A Connecticut Yankee should apear in American Literary Realism.  I
spent quite a bit of time with "The Private History of a Campaign That
Failed" while researching the essay.  Here's a thought that I didn't
mention in that essay, since I had no scholarly poof of it, but of which
I am relatively sure:  "The Private History" is fiction.  I know that
Twain never admitted this.  Ostensibly it represents a true account of his
experiences.  It originally appeared in the CENTURY magazine as part of
the "Battles and Leaders of the Civil War" series, along with reminiscences
of well known soldiers and others.

But my gut tells me otherwise.  Again, I have no proof of this--as far as
I know, Twain never addressed the fictionality of the story overtly.
The tone of the piece, however, seems to me fictional.  The "stranger"
in the story seems less an actual person than a fictional substitute
for General Grant (Twain was working on publishing Grant's Memoirs at the
time he wrote the story; for more detail see my essay when it appears).
The shooting of the Union soldier does not ring true.  It reminds me of
Huck shooting his first bird:  an invented episode meant to evoke emotion.
My guess is that Twain's true Civil War experiences were much less dramatic.

I'm not trying to start a debate.  But I am interested in opinions
about this admittedly unsupported assertion.