Two years of bloody violence against a brutal dictatorship in Syria have awakened the ancient rivalry between Sunni and Shiite Muslims. It's become a religious battle, pitting neighbors against each other in Syria and across the Middle East, and is accentuating hostilities between nations, including Saudi Arabia and Iran. Terrorist organizations, like Shiite Hezbollah and Sunni al Qaeda, are threatening chaos to advance their competing agendas. Is it partly an unintended consequence of America's occupation of Iraq? Is there anything the US can do to contain it?