Troops have been carrying out public security duties for years as part of Mexico's war against drugs and the gangs which control their trade.It will still have to be passed by the legislatures of 31 states and the capital, Mexico City, to come into law, but the big hurdle was getting it approved in Congress.Before coming to office, Mr López Obrador was an opponent of having troops on the streets, and he campaigned on a promise to return them to their barracks.Once in office, he created a new civilian-led force, the Guardia Nacional (National Guard), to replace the federal police and to boost security in areas wracked by cartel violence.However, the United Nations called it a "setback to public security grounded in human rights", arguing there would be no civilian oversight of the National Guard."