BAGHDAD - While U.S. politicians and generals in Washington debate the possibility of civil war in Iraq, U.S. officers here and troops who patrol Baghdad daily say it has already begun. U.S. soldiers in and around Baghdad interviewed in the past week cite a long list of evidence that the center of the nation is unraveling: Villages have been abandoned by Sunni and Shiite Muslims; Sunni insurgents have killed thousands of Shiites in bombings and assassinations; Shiite militia have tortured and killed hundreds, if not thousands, of Sunnis. And neighborhoods have become open battlegrounds at night. "There's one street that's...