Iran and Iraq, though neighbors for many centuries, share both a common and a contentious history. Though both are Muslim nations, they have long been divided by their differing affiliations with the Shia and Sunni branches of Islam and by a cultural tension between Persian and Arab. These tensions have occasionally erupted into all-out warfare, most recently in the 1980s, when half a million Iraqis and Iranians were killed in a decade-long war. Today,...