It is understandable that many will not readily be persuaded of Iranian sincerity, not least because the national negotiating style – among moderates and hardliners alike – tends to be anything but frank and direct, with private reason often accompanied by public thunder that makes it hard to assess real intent. But the Vienna agreement deserves wide support, and not just because the alternatives – continuation of acute regional tension at best, and catastrophic military conflict at worst – are so unappealing.