mexico city mexico congress friday approve law enshrine use army country long war drug cartels override protest unite nations right body activists fear encourage abuse military rule institutional revolutionary party members center-right opposition national action party back bill head president enrique pena nieto desk sign law know law internal security bill establish rule military role battle drug gang conflict claim well 100,000 live last decade military mire several human right scandals include extra-judicial kill suspect gang members 2014 disappearance 43 students near army base supporters legislation say set clear rule limit use soldier fight crime right group convince say bill empower security force instead improve police could usher great abuse impunity unite nations amnesty international mexican human right organizations lobby lawmakers pass bill mexico enshrine army 's role drug war divisive law