Techniec
Drugs and Kalashnikovs
You're actually less objective than I am because you expect only Iran to behave rationally while i expect everyone to behave rationally. Iran WILL and IS behaving rationally which could lead to other nations behaving rationally as well. What happened in Pakistan is Indian built nuclear weapons and, RATIONALLY, Pakistan followed suit. Pakistan doesn't exert much influence because they have parity in the region with India but I would argue that do hold much more influence today than they did pre-nuclear.
Iran poses no threat to the US as far as relative power is concerned because we are far superior. But they do pose a threat to relative power in the region which could lead to other states behaving RATIONALLY in attempting to acquire nuclear weapons themselves to reach parity and hence sparking a nuclear race. What's to stop the countries in the region to not behave rationally? That's problematic in such a volatile region where we hold interests. Now you could argue that if that's a problem why not eliminate all nuclear weapons in the region(ie Israel). IMO, stable liberal governments are allowed nuclear deterrent. That's ideological and an ideology that is shared by most people in the US military what can I say.
israel is not a stable liberal govt though. and you use the domino theory (iran getting a nuke, leads to others) but that domino didnt start with iran, it already began with israel, pakistan, india and russian getting nukes. all of whom are in irans neighbourhood. irans policies are simply reactionary