REYNOSA, Mexico - An army convoy on the hunt for traffickers rolled out of its base in this border town under the control of the Gulf Cartel - and an ominous voice crackled over a two-way radio frequency to announce just that. The voice, belonging to a cartel spy, then broadcast the soldiers' route through the city, turn by turn, using the same military language as the soldiers.

"They're following us," Col. Juan Jose Gomez, who was monitoring the transmission from the front seat of an olive-green pickup truck, said with a shrug.

The presence of the informers, some of them former soldiers, highlights a central paradox in Mexico's ambitious and bloody assault on the drug cartels that have ravaged the country. The nation has launched a war, but it cannot fully rely on the very institutions - the police, customs, the courts, the prisons, even the relatively clean army - most needed to carry it out.

The cartels bring in billions of dollars more than the Mexican government spends to defeat them, and they spend their wealth to bolster their ranks with an untold number of politicians, judges, prison guards and police officers - so many police officers, in fact, that entire forces in cities across Mexico have been disbanded and rebuilt from scratch.

Over the past year, the country's top organized crime prosecutor has been arrested on suspicion of receiving cartel cash, as was the director of Interpol in Mexico. The cartels even managed to slip a mole inside the U.S. Embassy. Those in important positions who have resisted taking cartel money are often shot to death, a powerful incentive to others who might be wavering.

This was a war begun by Mexico, but supported - and in some ways undermined - by the United States.

American drug users are fueling demand for the drugs, and American guns are supplying the firepower wielded with such ferocity by Mexico's cartels - a reality acknowledged by Secretary of State Hillary Rodham Clinton on her trip to Mexico last week.

With the prospect of a quick victory increasingly elusive, a rising chorus of voices on both sides of the border is questioning the cost and the fallout of the assault on the cartels.

Mexicans, aghast at the rising body count, the mutilated corpses on their streets and the swagger of the drug chieftains, wonder if they are paying too high a price in pursuing organized crime groups that have operated for generations on their soil.

"Sometimes, I think this is a war you can't really win," a Mexican soldier whispered to a reporter, out of earshot of his commander, during a recent drug patrol in Reynosa. "You do what you can, but there's so many more of them than us."

Americans, including border state governors and military analysts in Washington, have begun to question whether the spillover violence presents a threat to their own national security, and, to the outrage of many Mexicans, whether the country itself will crumble under the strain of the war.

The impetus for the drug war began during President Felipe Calderon's 2006 campaign.

Although the economy was the No.1 issue, Calderon, a law-and-order technocrat, was paying attention to a steady rise in criminality early on. Calderon received threats on his life from drug cartels during the campaign, fueling his outrage, according to officials close to him. And he began to suspect that drug money was finding its way into political parties.

The U.S. Drug Enforcement Administration says Mexico's battle against drugs is clamping down on supplies, citing the doubling of cocaine prices in the United States over the past two years. But violence has gone up, not down. Although Mexicans have largely backed Calderon's efforts, the figure they seem most fixated on these days is the more than 6,200 drug-related killings in 2008, up more than 100 percent from 2007, and the more than 1,100 deaths so far in 2009.