The Islamic State’s summer rampage across northern Iraq sent thousands of Christians and members of other non-Muslim religions fleeing for their lives as members of the self-declared caliphate burned and desecrated Christian sites and ordered conversions to Islam. As the militants executed and enslaved Yazidi men, women and children, with thousands more fleeing up Mount Sinjar in the scorching heat, President Obama told the nation that the United States had to save a starving “ancient religious sect’’ from genocide.