The international nuclear protocol which states that members can only use nuclear energy for peaceful purposes, and not for weapons.
The nuclear power plant they are building, with Russian technical assistance, is consistently inspected by the IAEA. To date, no nuclear materials have been found to have "gone walkabout", nor have the Iranian scientists enriched their uranium above 20%, which is the level necessary to create medical isotopes. In order to create weapons, one has to enrich the uranium to above 90%, which flat out hasn't happened.
The IAEA has reported nothing to date to indicate that the Iranian government is building a nuclear weapon.
On the other hand, Israel, which has nuclear weapons, refuses to become a signatory to the NNPT, nor will it allow its nuclear sites to be inspected. Saudi Arabia's Prince Turki Al-Faisal is absolutely right to suggest that the US should encourage Israel to let go of its nuclear weapons; unfortunately, that's only going to happen when pigs fly.
But let's say, for the point of argument, that Iran is truly in the process of developing a nuclear weapon; in light of the fact that the US gave both Iran and the world a very concrete object lesson in what can happen to a country without a nuclear deterrent as it did with the invasion and occupation of Iraq, who can blame them?!?