is developing a nuclear weapon, looks at that and says, aha! the united states can be behind this. >> i think it's very difficult for us to know exactly what is happening in tehran. but what we do know is to communicate with them, we have to be very clear, very forthright. this is an opportunity to be both with the iranians to make sure that they understand that they do not have greater freedom of action. they do not have greater operating space to pursue a nuclear weapon which would destabilize that entire region, threaten our friends and allies and ultimately threaten us. >> is this more about iran than it is syria? >> this is about a series of very important things. in the first instance, almost 100 years old now, a prohibition against the use of chemical weapons. why does that matter? well, it matters for the reason that you started the show with, but it also matters for another reason. our troops have not been subjected to chemical weapons attacks since world war i.