After the war he was elected Senator from Missouri, served as Secretary of the Interior for Rutherford B. Hayes, and then retired to New York to write. He always had very strong opinions and no reluctance in expressing them, in letters, speeches, and articles that were forceful (if you agreed with him) or vitriolic (if you didn't).
Two things of his I've stumbled across seem worth quoting. The first is from remarks he made during debate in the Senate on February 29, 1872
The Senator from Wisconsin cannot frighten me by exclaiming, 'My country, right or wrong.' In one sense I say so too. My country; and my country is the great American Republic. My country, right or wrong; if right, to be kept right; and if wrong, to be set right.
The second is from the article, "About Patriotism" that he published in Harper's Weekly on April 16, 1898:
The man who in times of popular excitement boldly and unflinchingly resists hot-tempered clamor for an unnecessary war, and thus exposes himself to the opprobrious imputation of a lack of patriotism or of courage, to the end of saving his country from a great calamity, is, as to "loving and faithfully serving his country," at least as good a patriot as the hero of the most daring feat of arms, and a far better one than those who, with an ostentatious pretense of superior patriotism, cry for war before it is needed, especially if then they let others do the fighting.
Some things never change.