Non American, but I'll say, Bill Clinton.
Why? Because he was the first American politician to admit that Slovenia was right so seek independence. In 1991 when we fought our 10day war, the message from state department was quite clear, "We will never recognize you." It's only thanks to extensive lobbying from Germany that the EU recognized us, and following that, USA had no choice.
Bill Clinton was the first US politican to admit USA was wrong to deny another nation the right of self-determination.