BAGHDAD — The war in Iraq has entered a bloodier phase, with American casualties steadily declining over the past five months while the killings of Iraqi civilians have risen tremendously in sectarian violence, spurring tens of thousands of Iraqis to flee from mixed Shiite-Sunni areas. The new pattern, detailed in casualty and migration statistics and in interviews with American commanders and Iraqi officials, has led to further separation of Shiite and Sunni Arabs, moving the country toward a de facto partitioning along sectarian and ethnic lines – an outcome that the Bush administration has doggedly worked to avoid over the past three years The nature of the Iraq war has been changing since at least late autumn, when political friction between Sunni Arabs and the majority Shiites rose even as American troops began to carry out a long-term plan to decrease their street presence. But the killing accelerated most sharply after the bombing on Feb. 22 of a revered Shiite shrine, which unleashed a wave of sectarian bloodletting. About 900 Iraqi civilians were killed in March, up from about 700 the month before, according to the Iraq Coalition Casualty Count, an independent organization that tracks deaths. Meanwhile, […]