The biggest problem for the United States is not Iran getting a nuclear weapon and testing it, it's Iran getting a nuclear weapon and not using it. Because the second that they have one and they don't do anything bad, all of the naysayers are going to come back and say, "See, we told you Iran is a responsible power. We told you Iran wasn't getting nuclear weapons in order to use them immediately." ... And they will eventually define Iran with nuclear weapons as not a problem.
Wait, did people believe Iran wanted to get nukes in order to use them immediately? Who believed that? Do you know someone who believed that?