Iraq: No, we do not have weapons of mass destruction.
USA: Well, we don't believe you, so we're coming in to find them.
Iran: Hey guys, we're enriching uranium and should have enough for a nuclear weapon in about a year! We have up to 3,000 centrifuges doing the work.
USA: Well, we don't believe you. Since our President isn't very popular right now, we're not going to do anything to you.
But, look out for Ban Ki-moon. He's pissed, and he has the power to sanction you!