The US won't mess with anybody with the ability to fight back... 
Iraq 'might' have nukes (read; doesn't), US invades.
Iran is trying to create a civilian nuclear program, which would give them the ability to have nuclear missiles; US threatens to invade.
North Korea makes it clear that they want to make offensive nuclear weapons; US negotiates with the unspoken possibility of military action on the table.
Russia, which already has nuclear missles, invades a small country; US wags their finger.
The US won't intervene in this matter, I don't think. I certainly hope we don't, at least.
US has the strongest military in the world. A posting of a fleet with a Nimitz class carrier would make a huge difference.
The difference between North Korea and Iran is that the North Koreans are atLEAST, rational. They'll know when they should stop. Iran doens't care. It's how they were raised, with death as something not so bad. Iranians wouldn't be afraid to die at the hands of a nuke.
Iran isn't trying to create a civilian nuclear program. Face the facts, they've openly said that if they get nukes, Israel will be on the reciving end.
If we're lucky, Russians (and us) wont give the tactical nuclear weapons even a thought when it comes to using them.