VERACRUZ, Mexico - Five years after President Felipe Calderón launched an offensive against Mexico's five main drug gangs, 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 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 gangs. 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 identified for security reasons, told The Associated Press that the Zeta and Sinaloa gangs are now the nation's two dominant drug traffickers. One or the other is present almost everywhere in Mexico, and 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 Sinaloas, 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 state of Nayarit.

When Calderón took office in December 2006, he said the drug gangs were trying to take over the country. He launched the government's first broad attempt to fight them, deploying thousands of soldiers to capture members and dismantle the organizations.

At the time, the Zetas were not even a separate gang, but rather an enforcement wing of the Gulf Cartel, a role created in the late 1990s when they were recruited from an elite army unit. Sometime last year, 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 gangs, 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" drug gangs 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 Sinaloas are known for forging temporary alliances, officials believe the Zetas 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 gangs, with each seeking to exterminate the other.