As Iran sits down again today with five world powers — the US, the UK, France, Russia and China, plus Germany — there's a lot at stake for two leaders who've risked their legacies on the outcome. Iran's President Rouhani could see crippling economic sanctions lifted at last. President Obama could head off the possible threat of more nuclear weapons. But failure is likely to discredit both leaders with friends and foes in the Middle East, not to mention hardliners at home. We hear that Iran's increased transparency is cause for optimism, while intractable differences could make the world more dangerous than ever.