In a city long consecrated to the art of difficult conversation, Austria has closed its door to Iran's nuclear chief, refusing him entry to Vienna and prompting a formal protest from Tehran. The act is small in gesture but large in implication — a European nation signaling that the terms of engagement with Iran may be changing, and that the neutral ground diplomacy depends upon is not guaranteed. Vienna, home to the International Atomic Energy Agency and host to years of painstaking nuclear negotiations, now finds its own neutrality in question. What unfolds next will say much about whether di