The respective policies of the governments of Iran and Pakistan pose serious challenges to US interests in the Middle East, Asia, and beyond. These two regional powers, with a combined population of around 300 million, have been historically intertwined in various cultural, religious, and political ways. Iran was the first country to recognize the emerging independent state of Pakistan in 1947 and the Shah of Iran was the first head of state to visit the new nation. While this relationship shifted following the 1979 Iranian Revolution, and tensions do exist between Sunni Pakistan and Shi'i Iran, there has nevertheless been a history of cooperation between the two countries in fields that are of great strategic interest to the US: Afghanistan, nuclear proliferation, and terrorism. Yet much of this history of cooperation, conflict, and ongoing interactions remains unexplored. Alex Vatanka here presents the first comprehensive analysis of this long-standing and complex relationship.