GUADALAJARA, Mexico — Mexico's two most powerful criminal gangs are locked in a titanic battle for control of the country's heartland in a struggle that's redrawn Mexico's map of violence.

Violence has dropped along the U.S. border, with Ciudad Juarez, once considered the most violent city in the world, seeing a 35 percent decline in homicides this year.

That good news is balanced by bad news in Guadalajara, Culiacan and Veracruz, where the Sinaloa cartel, whose bulwark has always been Mexico's Pacific coast, and the Zetas, a violent gang that originally was created to protect the Gulf cartel along the Gulf of Mexico coast, are locked in a spiraling struggle that's seen each gang invade the other's territory.

The conflict has thrust Guadalajara, an important manufacturing center of 4.4 million people, into the battlefield. After overcoming a spate of drug violence in the mid-1980s, Guadalajara quieted down, perhaps because the Sinaloa cartel held a monopoly on operations in the surrounding state of Jalisco.

"Here in Jalisco, we've seen this as a distant thing. 'Oh, this is happening over in Michoacan.' It felt like it was far away," said Dante Haro Reyes, a law professor and public security expert at the University of Guadalajara. "Now it feels like it's around the corner."

The wakeup call came at daybreak last Thursday, when mobsters abandoned three vehicles filled with 26 dead bodies at the iconic bright-yellow Millennium Arches that straddle a Guadalajara thoroughfare. A message on a poster board was signed "Z," a signature of Los Zetas.

"Look how we leave you these dead people," the poster said in part. "We are in your kitchen."

Boasting of their penetration deep into Sinaloa turf, the Zetas claimed to be "the strongest cartel at the national level, the only cartel that doesn't pass information to the gringos," a reference to the son of a Sinaloa boss who claims to have been a Drug Enforcement Administration informant before his 2009 arrest.

Just a day earlier, the Zetas had dealt another blow to Sinaloa, leaving a truck filled with 16 charred bodies in Culiacan, the capital of Sinaloa state, from which the Sinaloa cartel takes its name.

The war between the groups — clearly the alpha dogs of Mexico's underworld — pits not just weapons but also two very different business models and geographic strongholds.

"This is a kind of death struggle, a definitive struggle between the Zetas, who have no remorse and expand constantly, and Sinaloa, which is trying to consolidate itself," said Bruce Bagley, an organized crime and narcotics expert at the University of Miami.

Sinaloa operatives appear to have set off the conflict over the summer, forming a group called "Matazetas," or Zeta Killers, to exterminate Zetas in Veracruz, a Gulf Coast state that's a bottleneck on a key smuggling route. The group went public in a big way at afternoon rush hour on Sept. 20, parking three vehicles packed with dead bodies near an urban underpass. Security agents found 35 victims at the grisly scene, nearly all asphyxiated and partially naked.

The "Zeta Killers" released videos of masked gunmen promising to hunt down Zetas and end their rampant extortion in Veracruz against common people.