This is a very old tactic.
Iran is using the American threat to try and reassert its control over its people.
It knows America will stop it becoming nuclear, but it keeps goading America because with the people concentrating on the external enemy they have less time to look at all the failed policies.
Massive unemployment, falling living standards, huge dissension.
America's greatest enemies remain her many traitors within Government (e.g., the leftist scum in the CIA and State Department), the media, and the universities.
We can survive nation building, however, no nation can forever survive with a dedicated enemy within. Cicero summed this up nicely.
I just hope that one day these folks are made to pay.