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After a meeting in Geneva that was the first fruit of President Obama’s policy of engagement on Iran comes a new encounter in Vienna. The meeting with Iran in the Austrian capital Monday is technical, with the goal of getting enriched uranium shipped out of the Islamic Republic. It is also an “Audacity of Hope” moment in foreign diplomacy, a potentially transformative development which few expected and most doubt is possible. The Iranians agreed, in principle, in Geneva on October 1 to send uranium that can be used to make atom bombs to a safe place outside of the country. This would reduce the threat that Iran could use the uranium for a nuclear weapon and give time for non-proliferation talks. The question Monday is: Will the deal go forward, collapse, or perhaps what is worse,...