The Defense Department confirmed Thursday that 2,500 US troops have died in the Iraq war since it began more than three years ago. The grim milestone was announced just hours before the House of Representatives was to begin a symbolic election-year debate over the war, with Republicans rallying against calls by some Democrats to set a timetable for the withdrawal of US troops. While there were no details on who it was or where the 2,500th death occurred, it underscored the continuing violence in Iraq, just as an upbeat President George W. Bush, freshly returned from Baghdad, determined that the tide was beginning to turn in the war. According to the Pentagon totals, there have been 1,972 service members killed in action in Iraq, and another 528 died from other non-hostile causes. There also have been 18,490 troops wounded in action, including 8,501 who did not return to duty.