Marx sent a letter to Lincoln, and Lincoln's staff responded via Ambassador Adams. It's a really interesting moment in history that's been buried by US Red Scare ideology.

from wiki on Adams:

Part of his duties included corresponding with British civilians, including Karl Marx and the International Workingmen's Association.[7] Adams and his son, Henry Adams, who served as his private secretary

https://www.marxists.org/archive/marx/iwma/documents/1864/lincoln-letter.htm