BAGHDAD, Feb. 21 — A truck bomb that combined explosives with chlorine gas blew up in southern Baghdad on Wednesday, and officials said it might represent a new and deadly tactic by insurgents against Iraqi civilians. It was at least the third truck bomb in a month to employ chlorine, a greenish gas also used in World War I, which burns the skin and can be fatal after only a few concentrated breaths. The bomb killed at least two people and wounded 32 others, many of them sent to hospitals coughing and wheezing, police and medical officials said. Iraqi and...