Two and a half years after the American invasion, deep divides that have long split Iraqi society have violently burst into full view. As the hatred between Sunni Arabs and Shiites hardens and the relentless toll of bombings and assassinations grows, families are leaving their mixed towns and cities for safer areas where they will not automatically be targets. In doing so, they are creating increasingly polarized enclaves and redrawing the sectarian map of Iraq, especially in Baghdad and the belt of cities around it.