
Since the Iraq War, sectarian conflict between Shiites and Sunnis has emerged as a major fissure in Middle East politics — fueling conflicts in Lebanon, Syria, Iraq, and Yemen; a resurgence of extremism and the scourge of the Islamic State; and an escalation in tensions between Iran and Saudi Arabia which has become the most significant clash between regional rivals in decades. From country to country, across the region, sectarian conflict is the thread that runs through each crisis, tying them into a strategic Gordian knot.