The U.S. has toppled 3 heads of state within the last 20 years, in Panama, Iraq and Serbia. That's what almost guarantees that North Korea and Iran will continue developing nuclear weapons. The U.S would not have invaded Iraq if it was a nuclear country. And no leader wants to end up like Saddam - or Milosevic, or Noriega.
That's what makes the final endgame of these confrontations murky. Will the United States initiate military strikes before either become full nuclear powers, and under what conditions ?