US opinion in the Franco-Prussian War was generally pro-Prussia; Bismarck's desire to unite Germany was received sympathetically by a nation that had undergone its own war for national unity a few years earlier, and remembered France's far from friendly role in that struggle. George Bancroft (US minister in Berlin) wrote in a letter to Secretary of State Hamilton Fish:
"Our foreign political interests almost always run parallel with those of Germany, and are often in direct conflict with those of France. Bismarck and the King were true to our union during our Civil War--when France took sides against us, Germany respected the independence of Mexico; the French supported the Austrian adventurer. Bismarck loves to give the United States prominence in the eyes of Europe as a balance to Great Britain. If we need the solid, trusty good will of any government in Europe, we can have it best with Germany; because German institutions and ours most nearly resemble each other, and because so many millions of Germans have become our countrymen. This war will leave Germany the most powerful state in Europe, and the most free; its friendship, is, therefore, most important to us ; and has its foundations in history and in nature. The more I learn of the present condition of France, the more deeply does the country seem to have been injured by the corrupting, wasteful and immoral government of Louis Napoleon."
But AFAIK nobody in the US advocated actual US intervention in the war.