Every Liberal in creation is saying the choice is between Obama's bad deal and immediate war with Iran. I think that's a false choice. It's possible that sanctions combined with the falling price of oil may seriously weaken the regime. Manufacturing and running centrifuges is expensive. The Iranians also have a huge and expensive security structure. They are sponsoring Shi'ite militias or armies in wars in Syria, Yemen and Iraq. They have expensive internal subsidies for food and gasoline to keep the populace under control. All of this has to be paid for by oil revenues. Estimates vary, but the Mad Mullahs need somewhere between $100 to $130 a barrel of oil to pay for all of this. Right now the price is about $55 a barrel. Iran has lots of natural gas reserves, but no export terminals and limited export pipelines. They will not be able to borrow money for export facilities if they are under sanctions. The US could put even more pressure on them by legalizing the export of crude oil and natural gas.
While I'm sure that the Mullahs won't cut the centrifuges, if they cut sponsoring wars and terrorism, their allies would be defeated. Defeats in foreign adventures can be deadly to the prestige authoritarian governments need for survival. If they cut in domestic subsidies they could be overthrown in the resulting unrest. If they cut domestic security they could lose control of dissidents and be overthrown. Add to the mix that Iran's Supreme Leader, Ali Hosseini Khamenei, has terminal cancer and is expected to die within 2 years. The combination of financial stress and a succession struggle could topple the whole regime. It's worth at shot. The current agreement guarantees an Iranian nuclear weapon within 10 years or less. That's a lock on a nuclear war in 10 years or less.