Barry Posen argues that it is not such a disaster if Iran gets nuclear weapons.
He says that there are three things that they could do.
First, Iran could give the weapons to terrorists. But he says that Iran will not do this because of what would happen to them if they did. He says a country that found out that Iran had provided the weapon that was used to attack them would attack Iran with many nuclear weapons. This threat will stop Iran from giving weapons to terrorists.
Second, Iran could blackmail countries with the threat of nuclear weapons. Posen says those countries would turn to the US for help rather than giving in.
Finally, Iran could use the weapons to deter anyone from attacking them. They could then go out and destabilize other countries with impunity. Posen says this won't work -- countries can reform themselves in ways that will prevent Iran from destabilizing them.