The US will never launch a nuclear attack on Iran. See, under the NPT, the US has promised that it will never use a nuclear bomb against a country like Iran. A country that does not even have a nuclear bomb. If the US does this, all the non-nuclear countries in the world will quickly start making nuclear weapons.
After 1945, the world had made the principle that nuclear weapons would not be used in wars. If the United States uses nuclear weapons against Iran, this principle would also be broken