I don't know what you mean by "defending" exactly, but if you wonder why people are sceptic on US-Iran relations, you must go back before 1979 - when a good ol' US buddy named Reza Pahlavi was brutally murdering it's citizens and US was in full support of his ruling, yet since he's gone US is on a 3-decades rampage of sanctions disturbing Iran's ability to function economically on the international scene.