WASHINGTON — As many as 200 U.S.-trained Mexican security personnel have defected to drug cartels to carry out killings on both sides of the border and as far north as Dallas, Rep. Ted Poe, R-Humble, told Congress on Wednesday. The renegade members of Mexico's elite counter-narcotics teams trained at Fort Benning, Ga., have switched sides, contributing to a wave of violence that has claimed some 6,000 victims over the past 30 months, including prominent law enforcement leaders, the Houston-area Republican told the House Foreign Affairs Committee. The slaughter has gained urgency amid high-profile assassinations of law officers in Mexico since...