Although the Mexican army has been able to quiet drug violence in some hot spots, political observers say the deployment of thousands of soldiers could undermine civilian institutions and jeopardize Mexico's evolving democracy.
Critics say the military lacks the training and sensibilities for such work, and fear it will trample on the rights of ordinary Mexicans.
The army, with its low salaries and high desertion rate, also could prove as vulnerable to corruption as police, who often have acted as hired guns for smugglers. Five Mexican soldiers, including a major, were indicted in January on charges of leaking information on their unit's movements to members of the Sinaloa drug cartel.
"The amount of money is huge," said Luis Garfias, a retired three-star general who said he fended off entreaties while stationed on the border in Mexicali in the early 1990s. "You like women? You like alcohol? It's free for you. Completely free, and dangerous."
During the 1980s, the army's job was mainly to find and destroy opium poppy and marijuana crops in western and northern Mexico.
In the 1990s, then-President Ernesto Zedillo ordered the air force to chase drug flights and named an army general as the nation's top anti-drug officer.
That general, Jose de Jesus Gutierrez Rebollo, was later convicted on charges that he helped Amado Carrillo Fuentes, reputed head of the Juarez cartel.
Zedillo's successor, Vicente Fox, maintained army involvement in the drug fight by naming a general as federal attorney general. But President Felipe Calderon, a conservative elected in 2006, has ratcheted up the military's role to new levels.
"The military for the first time is being used in a very blatant way to substitute for the incompetence and corruption of civilian agencies," said Roderic Camp, an expert on the Mexican military who teaches government at Claremont McKenna College in Claremont.
Nuevo Laredo, where gun battles and kidnappings chased fearful business owners out and kept tourists away, has quieted down since the army arrived. Many residents wonder whether the quiet will last; some believe it is more likely the result of an arrangement among drug rivals. But any respite is welcome.
"There haven't been clashes between armed men. There haven't been violent deaths," said Police Chief Alfonso Olvera, whose 517 officers were disarmed in January while the military ran a criminal check. "A big part of this is the work that the [army] is doing."
Jewelry shop owner Rogelio Armenta, who doubles as the city's point man on tourism, hopes to entice visitors back to Nuevo Laredo, where one-third of the tourist businesses shut down in recent years.
Some activists say the army, long accused of heavy-handedness in dealing with domestic dissenters, has shown the same tendencies in the drug fight. But polls show that ordinary Mexicans favor using the army.
In a surprising turnabout, Jose Luis Soberanes, the nation's human rights ombudsman, said this month that it would be "suicide" to pull the military from the streets. Soberanes had previously called for an end to the deployment.
Almost everyone agrees that there are few good alternatives to the army unless the government can improve the overall professionalism of the police, at least at the federal level. Past efforts have given way to more corruption, and more promises of reform.
Many analysts say the pattern will repeat itself as long as the U.S. offers a lucrative market for illegal drugs. What officials lack, critics say, is an exit strategy for the military.
"The biggest question we are going to face is, how are we going to pull the soldiers off the streets?" said Erubiel Tirado, a national security expert at the Ibero-American University in Mexico City. "We are contaminating the military, politically speaking."