Western companies eager to take advantage of the new nuclear agreement with Iran face a host of challenges. One of the biggest: Iran’s Revolutionary Guard Corps. A branch of Iran’s military, the force is charged with defending the country’s Islamic system. During years of sanctions and economic isolation , it has also established itself as one of the country’s most powerful economic forces. Through an opaque-but-vast network of companies it controls, the Guards have interests in construction, manufacturing, banking and shipping, among other industries, said people familiar with the organization. Khatam al-Anbia, an engineering and construction firm at the heart of the Guards’ business empire, estimates the long-term value of its contracts is about $50 billion, equivalent to around 12% of Iran’s gross domestic product. If Iran implements the terms of the deal reached with six world partners in mid-July, many sanctions could be lifted by early next year. […]