About 3,300 Mexican soldiers, sailors and federal police arrived in Tijuana - a major centre for drug trafficking - on Tuesday to help tackle criminal gangs and corruption.
"Operation Tijuana" is the second major military offensive against drug gangs by Felipe Calderon, the Mexican president, who took office on December 1 promising to crack down on organised crime.
Last month, Calderon sent 7,000 troops to his native state of Michoacan in western Mexico, which has seen execution-style killings and beheadings as rival gangs have fought over marijuana plantations and smuggling routes.
Drug gangs were blamed for more than 2,000 murders across Mexico in 2006.
In Tijuana, federal investigators believe that a corrupt network of police officers has been supporting smugglers who traffic drugs across the nearby border into the United States.