BAGHDAD, Iraq - While American politicians and generals in Washington debate the possibility of civil war in Iraq, U.S. officers and enlisted men who patrol Baghdad daily say it has already begun. Army troops in and around Baghdad interviewed in the last week cite a long list of evidence that the center of the nation is coming undone: Villages have been abandoned by Sunni and Shiite Muslims; Sunni insurgents have killed thousands of Shiites in car bombings and assassinations; Shiite militia death squads have tortured and killed hundreds, if not thousands, of Sunnis; and when night falls, neighborhoods become open...