Iran, unlike the United States with a world-class military, will actually use the nukes if they get them - they said they would to ‘wipe Israel from the map’ in so many words. With a world-class military the conventional option is most probable to solve any scrimish before the nuclear option will be used.  Because the United States military is so strong its very unlikely that the United States will ever use nukes on Iran.

Since Iran actually support terrorist organizations (recently Taliban in Afganistan, Hezbolla in Lebonon, and terrorists in Iraq), the west fears that they will arm them with nuclear material to do their attack by proxy.  If a nuclear attack (ala 24), or dirty bomb (a conventional explosion with nuclear material mixed in), did occur many in the world would not advocate retribution to Iran if it was done by any of their proxy armies.

I mean how hard would it be to have Iranian terrorists enter in from Mexico? Middle easterners are being caught with Hispanic illegal aliens on the border with more and more frequency. Iranians with their tan skin can also blend into the Hispanic population easier.

Also with Chavez being buddy buddy with Iran there are fears that Chavez will help Iran train terrorists on islands off the coast of Venezuela to attack United States interests.

We also believe that Iran will bully its neighbors with nukes. What if Iran, for example, treatened Saudi Arabia to stop selling oil to the United States? You see?

These are some reasons why we don’t want Iran to get the nuke. I don’t think we would have a problem if it was a country like Denmark, Italy or even Canada.