The threshhold of nuclear war by the U.S. is too great---even Reagan told his close confidants he did not know if he could retaliate with nukes if the U.S. was attacked. Moreover, the fallout winds would sweep both Iraq and Afghanistan, endangering hundreds of thousands of U.S. troops. It won't happen.
"Iran can not have had time to sufficiently both develop nukes and protect them in impregnable bunkers."
"impregnable", hard to say. But the prized possessions of their nuclear projects are indeed deep underground.