USA, on the Arizona side. Pillboxes were actually built at this site, remote from the Pacific coast, during WW II! I guess people honestly worried that the Japanese might pull a Pearl Harbor here. In retrospect, of course, it was a gross overreaction.
UPDATE: Now that I know how to add inserts, I've included a view of the dam itself, as seen from the new bypass bridge downstream, with the position of this pillbox highlighted.