Here’s the story of contemporary Iran in a nutshell: A decade ago, Iran was relatively stable, led by a reformist president, Hassan Rouhani, who’d just signed a nuclear agreement with half a dozen world powers, including the United States. Its economy, long plagued by economic sanctions imposed most emphatically by the United States, began to expand as softened sanctions allowed its oil exports to grow, and its currency, the rial, traded at 32,000 to the dollar.