Looking at what Iran has done so far...

...and making conjectures on what it might do in the future. From Newsmax:

U.S. Intel: Iran Plans Nuclear Strike on U.S.
Iran has carried out missile tests for what could be a plan for a nuclear strike on the United States, the head of a national security panel has warned.

In testimony before the House Armed Services Committee and in remarks to a private conference on missile defense over the weekend hosted by the Claremont Institute, Dr. William Graham warned that the U.S. intelligence community �doesn�t have a story� to explain the recent Iranian tests.

One group of tests that troubled Graham, the former White House science adviser under President Ronald Reagan, were successful efforts to launch a Scud missile from a platform in the Caspian Sea.

�They�ve got [test] ranges in Iran which are more than long enough to handle Scud launches and even Shahab-3 launches,� Dr. Graham said. �Why would they be launching from the surface of the Caspian Sea? They obviously have not explained that to us.�

Another troubling group of tests involved Shahab-3 launches where the Iranians "detonated the warhead near apogee, not over the target area where the thing would eventually land, but at altitude,� Graham said. �Why would they do that?�

Graham chairs the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack, a blue-ribbon panel established by Congress in 2001.

The commission examined the Iranian tests �and without too much effort connected the dots,� even though the U.S. intelligence community previously had failed to do so, Graham said.

�The only plausible explanation we can find is that the Iranians are figuring out how to launch a missile from a ship and get it up to altitude and then detonate it,� he said. �And that�s exactly what you would do if you had a nuclear weapon on a Scud or a Shahab-3 or other missile, and you wanted to explode it over the United States.�

A largish (20MT) bomb exploded at 300 miles over Kansas would affect the entire continental US. I don't think that is within Iran's capability and if they did launch and if the bomb misfired (remembering North Korea's almost-dud), the reprisal would be swift and brutal. A blast over NYC down to D.C. would be a delightful realignment of the United States would seriously cripple our economy and the point of the article is that the Iranians do have this capability. You cannot reason with a madman who knows that the end time is near and that allah will sort everything out afterwards...