Five years after President Felipe Calderon launched an offensive against Mexico's five main drug cartels, the nation is now dominated by two powerful organizations that appear poised for a one-on-one battle to control drug markets and trafficking routes.
The government's success in killing or arresting some cartel leaders has fractured most of the other gangs to such an extent that they have devolved into quarreling bands, or been forced to operate as subsidiaries of the two main cartels. That has often meant expanded territory and business opportunities for the hyper-violent Zetas and drug lord Joaquin "El Chapo" Guzman's Sinaloa cartel.
"They are the two most successful cartels, or at least they have been able to expand in recent years," said drug trade and security expert Jorge Chabat.
Mexican federal authorities, who asked not to be named for security reasons, told The Associated Press that the Zeta and Sinaloa cartels are now the nation's two dominant drug traffickers. One or the other is present almost everywhere in Mexico, but officials are braced to see what happens next in a drug war that has already claimed an estimated 35,000 to 40,000 lives. So far, the signs are not hopeful.
In the Gulf coast seaport of Veracruz, 35 bound, tortured bodies were dumped onto a main thoroughfare during the height of rush hour on Sept. 20. The killers are presumed to be aligned with the Sinaloa cartel, while the victims were apparently linked to the Zetas, who took hold of the important seaport in 2010. In a clash in May, more than two dozen people, most of them Zetas, were killed when they tried to infiltrate the Sinaloa's territory in the Pacific Coast state of Nayarit.
When Calderon took office in December 2006, he said the drug cartels were trying to take over the country. He launched the government's first broad attempt to fight the gangs, deploying thousands of soldiers to capture cartel members and dismantle the organizations.
At the time, the Zetas were not even a separate cartel, but rather an armed enforcement wing of the Gulf cartel, a role created in the late 1990s when they were recruited from an elite army unit. Sometime around 2010, after a falling-out between Gulf and Zeta gunmen, the Zetas split off, ushering in what is possibly the bloodiest chapter of Mexico's narco wars. Within less than two years, the Zetas had taken control of the seaport and most of the Gulf's former territory.
According to Chabat, the two have survived the government crackdown because they have been more skilled than their weaker counterparts. He said the new alignment may make it easier for government forces to target the two big cartels, as opposed to fighting half a dozen of them.
"The question is whether the Sinaloa cartel and Zetas are going to break at some point or not," said Chabat.
"Right now they are very strong, but if in two or three years these cartels are pulverized, they may say that (the drug war) was a success."
Both the "mega" cartels want to control seaports for shipping drugs from South and Central America, and border towns, for getting the drugs into the United States.
Sinaloa has long been based on the country's northwest Pacific coast, with occasional incursions farther east along the border. In recent years, it has spread both east and south, reaching into Central America.
The Zetas, once confined to a stretch of the northern Gulf coast, have grown the most, pushing into central Mexico, and as far south as Guatemala.
Strategies differ. While the Sinaloa cartel is known for forging temporary alliances, officials have said the Zetas are believed to scorn them, preferring direct control of territory. There appears little chance the two groups will ever agree to split their turf; instead, Mexico may be headed into a battle between the two cartels, with each seeking to exterminate the other.
"I see the Sinaloa Federation and the Zetas as being the two polarizing forces in the Mexican criminal system ... and between the two, an array of other smaller groups aligned with one or the other, " said Samuel Logan, director of Southern Pulse, a security consulting firm.
Their operations differ too. The Zetas are involved in human trafficking and other illegal businesses, as well as the drug trade. They have committed some of the worst massacres in the Mexican drug wars and engage in a violence so brutal authorities have called the cartel "irrational." The Sinaloan hit men, on the other hand, appear to be more focused on the drug business and are less randomly violent.
Zetas often dress in fake military gear, and have erected military-style training camps. Sinaloa gunmen, like other narcotics gangs, are more discreet, favoring ski masks and black clothing.
"Sinaloa has done well by flying under the radar. They're comparatively less violent, though they're no saints," said Andrew Selee, director of the Washington-based Mexico Institute. "The Zetas have certainly gotten bigger since they split with the Gulf, but whether that will amount to a long-term ability to control and defend the territories where they have a presence is a little less clear.
"In reality, they're much thinner, where Sinaloa is hierarchical and compact."
Both the big cartels have also been known to launch "spoiler" attacks, aimed at making trouble on an opponent's turf, even though they have little chance of truly encroaching on it. They have sometimes even launched "poison" attacks on civilians on an opponent's turf, hoping the rival will be blamed.
In between the two giants, smaller, fragmented remains of vanquished cartels fight their own bloody battles.
On the outskirts of Mexico City, the Knights Templar cartel appears to be fighting remnants of the Beltran-Leyva gang, and the same two forces -- plus the Zetas -- have been battling for Acapulco, terrorizing the Pacific coast resort.
Battles among various cartels proliferate in Mexico's most violent cities, including Monterrey, where the Gulf cartel is fighting the Zetas.
But Selee notes that the Veracruz fighting may represent a new stage in which the two big gangs take each other head-on as they move deeper into each other's territory. The battle may have opened in May, when the Zetas apparently sent a convoy of fighters into Sinaloa territory in the Pacific coast state of Nayarit.
For all of the Zetas' bloody reputation -- they have been known to massacre the families of police or soldiers who had already died fighting them -- the incursion didn't go well: Twenty-eight presumed Zetas were found slaughtered by the side of a highway.
Soon after, in July, a group of two dozen armed men posted a video on the Internet, identifying themselves as "Mata Zetas" -- literally, "Zeta Killers" -- and said they were from a group allied with Sinaloa to hunt Zetas.
A Mexican military official who could not be quoted by name for security reasons said that besides the tit-for-tat aspect of the Veracruz killings, Sinaloa may also want control of the port as a link in the shipping route from Central America.
But Logan sees another reason for a group aligned with Sinaloa to attack deep into Zeta territory in Veracruz -- to distract the Zetas from their next target: Guadalajara.
Mexico's second-largest city also has seen a rise in drug violence in the past year. It was long the home of Sinaloa's methamphetamine-trafficking arm run by Guzman lieutenant Ignacio "Nacho" Coronel, who was killed in a shootout with federal police in July 2010. Since then, factions of Coronel's operation have been fighting for control, including the New Generation and another group known as the Resistance.
The Zetas have taken over neighboring Zacatecas state in their push west, and are eyeing Guadalajara both for the meth trade and for extortion potential.
"The Zetas aren't good for business. They do what they have to because they don't have the distribution networks of the Gulf or Sinaloa. So they have to diversify into kidnapping and extortion," said a U.S. law enforcement official in Mexico, who couldn't be identified for security reasons.
Logan said there are rumors that some factions fighting the New Generation are ready to join with the Zetas.
"That's got to concern El Chapo," he said, of the Sinaloan leader. "Guadalajara has been a huge part of the meth trade for years, El Chapo's bread and butter. If the Zetas take that, it won't be good for El Chapo."
Both big cartels are trying to cover their actions with public relations campaigns, as is now customary. The Zetas hung banners in several Veracruz towns, accusing the military of rights abuses and favoring Sinaloa.
The Mata Zetas have come out with another video, in which they claim to have moved into Veracruz to protect the public from Zeta kidnappings and extortion. The men's demeanor and language evoked a military style more than that of a gang foot soldier, raising a specter of a paramilitary response.
"We are the armed wing of the people, and for the people," says a man with a ski mask, who is seen in the video sitting at a table reading from a prepared statement. He is flanked by four other masked associates, each with a full water bottle placed on the tablecloth. "We are anonymous warriors, faceless, but proudly Mexican."
The Associated Press contributed to this report.