China agrees to Iran sanctions

China says it will support sanctions on Iran. This appears to be a very impressive piece of diplomacy by the Obama administration to get first Russia and now the very recalcitrant China to sign on to sanctions against Iran. I wonder how China was persuaded. Nothing fundamentally has changed recently that would move China toward sanctions. So what were the back room negotiations like? Did the US offer something in return to China? (as many people saw the end to the Star Wars missile defense systems with Russia as a way to get the Russians on board with Iran sanctions).