OK, so technically he's an alien, but really, what's more American than Superman? Heck, even his outfit is red and blue. But after years of fighting for "truth, justice, and the American way," the Man of Steel announced he's going to renounce his US citizenship in Action Comics #900. It all started when he independently and non-violently supported protesters in Iran, prompting the Iranian government to view his actions as an act of war from the United States.
Superman, realizing it was foolish to assume his actions wouldn't reflect on the US government, then declares his plan to renounce citizenship in order to go on working as a more global superhero. On Comics Alliance, Laura Hudson notes that Superman doesn't seem to be abandoning his traditional values, just "trying to implement them on a larger scale and divorce himself from the political complexities of nationalism." (Read more Superman stories.)