Oliver Wendell Holmes, like Adams a Union combat veteran, ended up sharing an opinion somewhat similar to Charles Francis Adams Jr.
I couldn’t locate the quote of his that I was looking for just now but here’s another touching on the war:
“We believed that it was most desirable that the North should win, we believed in the principle that the Union is indissoluable, we, or many of us at least, also believed that the conflict was inevitable, and that slavery had lasted long enough. But we equally believed that those who stood against us held just as sacred conviction that were the opposite of ours, and we respected them as every men with a heart must respect those who give all for their belief.”
"Between two groups of people who want to make inconsistent kinds of worlds, I see no remedy but force." - Oliver Wendell Holmes, Jr.
"The only prize much cared for by the powerful is power." - Oliver Wendell Holmes, Jr.
"A new untruth is better than an old truth." - Oliver Wendell Holmes, Jr.
"For my own part, I often doubt whether it would not be a gain if every word of moral significance could be banished from the law altogether…We should lose the fossil records of a good deal of history and the majesty got from ethical associations, but by ridding ourselves of an unnecessary confusion we should gain very much in the clearness of our thought." - Oliver Wendell Holmes, Jr.
Holmes got a reputation as a liberal idealist and reformer, but he was more a nihilist. Some people blame his skepticism or cynicism about moral values on the war, but Darwin had a lot to do with it.