Nah, I don't think that Iran when they develop their nuke, could reach the US conventionally. But, being a terrorist nation state, they could develop a method to deliver a nuke to a US city through a homicide bomber. Although, also not likely in the immediate, the more plausible threat is to hold this nation hostage through regional destabilization, and threat to Israel.
In any case it would not be a positive to adopt a blind eye, and just let Iran develop one. Sanctions may be an act of war, but they are the method used to avoid bloodshed. Problem is that they rarely work.