BAQUBAH, IRAQ-U.S. military officials here have little trouble knowing when al Qaeda is exerting its influence in this city that was, once upon a time, the lush, orange-growing capital of the Mideast. Sometimes it's the subtle signs: Tomatoes and cucumbers start disappearing from the market, deemed too sexually suggestive, soldiers here say, by the Sunni fundamentalist terrorists. On other occasions, it's more overt: "Graffiti that says, 'Long live al Qaeda,' and stupid stuff like that," explains a U.S. military officer based here in what is now the capital of Diyala province. Mostly, though, the pressure has been bold and deadly:...