The sectarian dimensions of today’s Middle East, with its active Sunni-Shia fault line, are more pronounced than they have been in centuries. Whether Iraq’s switch from a Sunni-minority regime to Shia-majority rule is a cause or a symptom of the current regional churn, it is clear that the sectarian divide has underpinned the crises in Iraq, Syria, Bahrain, Yemen, and parts of Saudi Arabia.