They could declare that if the US interferes with their invasion of say (a newly democratic) Syria, then the Iranians will nuke Berlin. The US says if you do that we will nuke Tehran and Mashhad. The Iranians declare an evacuation of those cities and continue with invasion and tell the US that they are prepared to pay that price. Is the US prepared to "pay" Berlin to defend Syria? Probably not.
I try to stay as far away from hypotheticals like this as possible, but we're supposed to spot you the evacuation of a city of over eight million, more than twice the size of Berlin? You really think the residents of Tehran and the Iranian people would be OK with this whole scheme any more than those of Berlin and the people of US and Europe would be?
I'm not saying that it wouldn't be a Bad Thing if Iran got nukes, but the idea that the worst things that could happen because of this involve tussling over Berlin, of all cities, make me think that someone forgot what decade they're in.