Iran Doesn't Have a Nuclear Weapon - Yet

But America does, along with Russia, Britain, France, Israel, Pakistan, India, North Korea and China.

Jonathan Schell book, “The Seventh Decade: The New Shape of Nuclear Danger,” discusses nuclear proliferation and how the Bush administration has ushered in a new nuclear era and encouraged other countries to pursue weapons of mass destruction.
Watch or listen to an interview of Schell on Democracy Now talking about the two biggest threats humanity faces today: nuclear weapons development and climate change.
Here's an excerpt of a response to Amy Goodman's question about what would happen if the U.S. did drop a nuclear weapon.

"...there is no city in the world that can’t be annihilated with a nuclear weapon of the appropriate size. And, you know, it occurs in many ways. First is the tremendous flash of heat, called the thermal pulse, which radiates out perhaps ten miles in every direction, setting fire on everything that it touches, that it reaches, and creating a firestorm. Second, you have the blast. And finally, you have the radiation. So it’s a kind of overkill exists, even within a single nuclear explosion, so to speak...just imagine even one—not to speak of three or—I mean, if it’s a hundred, then countries and continents are gone. Imagine any country on earth without its top hundred cities. If it’s one or two weapons, then you have a political convulsion of the first order, and you’d have to ask first: if it was a country that enjoyed liberty, would those liberties survive? I think they’d be gone the next day. I think that would be the first casualty. You’d have to wonder whether the government itself would survive under those conditions. So, you’d be in a different and unpredictable world.

Schell says Reagan and Gorbachev were both for nuclear disarmament. It's a good interview.
For more information about the nuclearization of the world, check out Beyond Nuclear, a site by Dr. Helen Caldicott, a leading figure in the disarmament movement and founder of the Nuclear Policy Institute.