The Nuclear Non-Proliferation Treaty (NPT) forbids any of it signatories to have nuclear weapons. Full stop.
If Iran is allowed nuclear weapons capability, other nations -- especially throughout South America, already infiltrated by Iran -- will doubtless follow suit.
Iran also happens to be the country with which North Korea most cooperates on ballistic missile development.
As the current U.S. Administration said it would take Israel's Prime Minister, Benjamin Netanyahu "at his word," it is, of course, safe to assume that it will take Iran's Supreme Leader at his word, as well.