Iran's historic reentry into global markets Saturday came with expected fanfare and protest, and few surprises. Least surprising of all, the implementation of the sanctions accord means that the toughest work for banks could lie ahead. As outlined by the Joint Comprehensive Plan of Action signed in July, the European Union and other jurisdictions lifted the bulk of economic restrictions targeting Iran, clearing the way for trade and investments in Iranian energy, automotive and infrastructure projects. U.S. officials also rolled back measures aimed at central banks and foreign firms working with Iran. But for financial institutions and other companies with...