What exactly, has the US got to fear from Iran`s nuclear weapons program?

A nation we don't control.

Iran will use them against Isreal or maybe as a terrorist act against the United States to try to disrupt the US economy. Have you heard Amidenijihad talk? He is bent on bringing on the apocolypse and I think he will try something crazy. If people were not so sensitive to terrorist attacks we would have much less to fear. Look at the aftermath of 911. If there were a nuclear explosion in the United States our government might get rid of all civil liberties. As for Isreal a Nuclear weapon will allow instant genocide.

Fortunately I think more moderate politicians will come into power in Iran or at least exert some influence there before they have Nuclear weapons.

mushroom clouds...we learned alot after europe tried appeasement to the nazis in the forties that resulted in its almost total destruction.