The war with Iran revealed just how much influence the world’s biggest oil importer has over prices and other countries’ supplies of jet fuel, gasoline and diesel. One of the most important lessons from the war with Iran is one that few saw coming. The conflict has made clear that China, which has long been highly vulnerable to oil supply shocks, has managed to transform that weakness into a surprising source of geopolitical power. The country made that big shift by stockpiling huge amounts of oil, building extra oil refining capacity and investing in coal and other alternatives. And in...