Thousands of U.S. troops swept house-to-house through mostly Shiite areas virtually unopposed Wednesday in the opening phase of the long-awaited Baghdad security crackdown. But four U.S. soldiers were killed outside of the capital in an area not covered by the operation. Elsewhere in Baghdad, Iraqi soldiers and police set up new checkpoints across the city of 6 million people, snarling traffic and forcing people to walk across bridges jammed with cars and trucks. Full Story