U.S. and Mexican officials describe the drug cartels as a widening narco-insurgency. The four major drug states average a total of 12 murders a day, characterized by ambushes, gun battles, executions and decapitated bodies left by the side of the road. In the villages and cities where the traffickers hold sway, daily life now takes place against a martial backdrop of round-the-clock patrols, pre-dawn raids and roadblocks manned by masked young soldiers.

Calderón's deployment of about 45,000 troops to fight the cartels represents a historic change. Previous administrations relied on Mexico's traditionally weak police agencies to combat the traffickers, who funnel 90 percent of the cocaine that enters the United States. The cartels corrupted local authorities and reached tacit agreements with the national government, limiting the violence while the drugs continued to flow.

After Calderón became president in December 2006, he told Mexicans that the use of the military against the cartels would be limited and brief. But it is now the centerpiece of his anti-narcotics strategy, according to interviews with senior U.S. and Mexican officials and dozens of people on the front lines of the war.

"It can be traumatic to have the army in control of public security, but I am convinced that we don't have a better alternative, even with all the risks that it implies," said Monte Alejandro Rubido, a senior public security official who is overseeing the overhaul of Mexico's police forces.

The military's withdrawal is dependent on the success of the police reforms, according to the government. U.S. and Mexican officials predict that troops will be patrolling the streets for years. In many regions, the army has become the law. But rather than quelling the violence, it increasingly appears to have been drawn into a deepening morass of cartel rivalries, local political disputes and blood feuds.

In the southern state of Guerrero, the army ratcheted up security last year, killing several alleged drug traffickers and making dozens of arrests. That was followed by a two-month stretch in which nine soldiers were abducted and decapitated in the state capital, four policemen were incinerated in a daylight grenade attack near a beach resort and a former mayor was shot 24 times before 1,000 people packed into a plaza for the coronation of a town beauty queen.

Mexicans have greeted the unprecedented deployment of federal troops in their communities with a mix of gratitude and dismay.

"There are a lot of opinions. I personally feel more secure to see the army out in the streets," said Denis González Sánchez, a 29-year-old city administrator in Petatlan, a Guerrero beach town of 30,000 where the army began patrols last year after three dozen gunmen massacred the family of a former mayor accused of links to traffickers. "A lot of people feel exactly the opposite: They say that the army is making us less secure. But I always think it's better knowing that they are out there protecting us, that they are watching over us, when there is nobody else to do it."

Mexican officials say the cartels operate on a $10 billion annual budget earned from drug sales in the United States; according to U.S. government estimates, they employ 150,000 people. This year, the Mexican government will spend $9.3 billion on national security, a 99 percent increase since Calderón took office.

Since December 2006, more than 10,100 people have been killed in the strife, including 917 police officers, soldiers, prosecutors and political leaders, according to Milenio, a Mexican media organization. At the same time, human rights complaints against the army have surged 576 percent, according to Mexico's National Human Rights Commission, including allegations of unlawful detentions, forced disappearances, rape and torture.

Calderón and his advisers have described the military's deployment as an emergency measure while he seeks to reform Mexico's local, state and federal police forces. He has promised that when the new police forces are ready, the troops will return to their barracks. That process may take until the end of his six-year term in 2012, he said recently.

The government is attempting to vet and retrain 450,000 officers, most at the state and municipal levels, employing lie detectors, drug tests, psychological profiling and financial reviews to weed out corruption and incompetence. Nearly half of the 56,000 officers vetted so far have failed.

The government is also forging agreements with each of Mexico's 31 states and its federal district, Mexico City, for the military to deliver automatic rifles, high-caliber ammunition, grenade launchers and fragmentation grenades to state and municipal officers who obtain federally mandated security clearances. "I can't hand over modern weapons systems to a police officer who has not fulfilled all the requirements," said Heriberto Salinas, a 70-year-old retired army general who commands the Guerrero state police force. "It has to be someone who is vetted and evaluated."

Mexican authorities are increasingly turning to retired army officers to run the police, counting on their discipline and training to resist the corrupting influence of the cartels and their ties to the military to help coordinate joint operations. In addition to Salinas, who came out of retirement at the request of Guerrero's governor, six of the state's eight operations coordinators are former military. At least a dozen governors have tapped retired generals as state police commanders, and hundreds of former military officers are serving at the municipal level. The assistant secretary for strategy and intelligence for Mexico's federal police, Javier del Real Magallanes, is an active-duty general.

Last month, Calderón dispatched an additional 5,000 troops to the border city of Ciudad Juarez, and the army took control of the police department after traffickers forced the resignation of the police chief by threatening to kill one of his officers every 48 hours.

Anthony P. Placido, chief of intelligence for the U.S. Drug Enforcement Administration, called Calderón's decision to use the military an "extraordinarily courageous step."

"This was not a traditional law enforcement problem that could be solved using traditional tools," Placido said in an interview in Washington. "It had gotten away from them. If the D.C. police were to engage in an operation against these criminal adversaries, and they faced bands of 30 to 50 of these criminals, and they were all carrying AK-47s and grenades and the bodies were dropping at the rate they're dropping, I suspect you might have to call in the National Guard. I don't think it is drastically different from what we would do if faced with a similar situation."

U.S. and Mexican officials familiar with Calderón's thinking said a confluence of events pushed him to declare war on the traffickers. During the presidential campaign, U.S. and Mexican officials independently received a tip that the powerful Gulf cartel had taken out a contract on Calderón's life, according to a source with direct knowledge. Although the tip was never verified, it was taken seriously because of the cartel's links to the Zetas — feared Mexican Special Forces veterans who served as assassins for the organization. U.S. authorities believe that the Zetas have broken off to form their own cartel in recent months.

The U.S. and Mexican governments also received information that drug money — one American official estimated $5 million to $10 million — made its way into the 2006 mayoral and parliamentary races. Calderón and his advisers viewed the influence of such money as a serious threat to democracy in the country, which had been governed for decades by a single political party — the Institutional Revolutionary Party, or PRI — until Calderón's predecessor, Vicente Fox, was elected in 2000.

"It was a factor that was considered, especially the municipal elections, because from there they could gain control of the local police forces," Eduardo Medina Mora, Mexico's attorney general, said in an interview. "This was why we couldn't wait. The threat was assessed from many different factors, this one being a relevant one, but not the only one."

Between Calderón's election in July and his inauguration in December, a surge in drug violence signaled the savagery to come. In September, a heavily armed gang apparently affiliated with the drug militia La Familia burst into the Sun and Shade disco in Uruapan, a favorite haunt of dealers in Michoacan, Calderón's home state. The men tossed five severed heads onto the dance floor, leaving a note that read, "Everyone should know, this is divine justice."

"One of the most critical elements in the decision to use the military was the amount of violence between the election and when we took over," a senior Calderón adviser said. "The executions, the decapitations, the confrontations between the drug gangs. There was a perception in society of lawlessness, that there was no state."

Calderón had campaigned on a law-and-order platform, but he never explicitly told Mexicans he intended to use the military. Traditionally, law enforcement has sought to contain, rather than to directly assault, the drug cartels in Mexico. Fox had targeted cartel leaders, a strategy that did nothing to eradicate the organizations.

People who have followed Calderón's career say the challenge played to his black-and-white view of the world and his deep-seated patriotism. The president, a Catholic with three young children, is known to have a sense of moral anger against the corrupting influence of drugs in Mexico, where consumption is a growing problem.

"He liked the mano duro," or iron fist, said Raúl Benítez, a national security analyst in Mexico City. "And in Mexico that means the military."

The army was responsible for one of the most painful episodes in recent Mexican history: the massacre of hundreds of student protesters days before the 1968 Mexico City Olympics. The military has gradually improved its public image since then by providing support during natural disasters such as hurricanes and earthquakes. It also fought small-scale guerrilla uprisings in the 1990s in the southern states of Chiapas and Guerrero. U.S. and Mexican officials said Calderón did not anticipate the violence his strategy appears to have unleashed. José Luis Piñeyro, a military analyst in Mexico City, said the president and his advisers had "launched a war for which they were unprepared."

But Calderón has given no sign of backing down. He said recently that drug trafficking is "a cancer that has invaded everything. So what you have to do is eradicate this disease, expose it to radiation and attack it. And of course this is expensive and painful, but it must be done."

Placido, the DEA intelligence chief, said he believes Calderón "is way past the point of no return. . . . This is my personal opinion, but I think he's all in. He has to fight to save himself, his party and his country."

Guerrero, on Mexico's Pacific coast, is a crossroads for drug traffickers and a laboratory for Calderón's policies.

The state is Mexico's largest producer of opium poppies, which flourish in its rugged highlands, and the second-largest producer of marijuana. Its 200-mile shoreline is a main transshipment point for Colombian cocaine entering Mexico en route to the United States. The coastal highway is a two-lane road between the resorts of Acapulco and Zihuatanejo that runs through fishing villages and past lush mangroves and breathtaking beaches. It is also one of the country's busiest drug-trafficking corridors.

Guerrero is convulsed by violence. The Mexican army's 9th Military Region has deployed about 5,000 troops to fight traffickers from Acapulco to the smallest mountain villages. At least four separate drug cartels are warring over territory, a mix made even more volatile by the presence of corrupt police forces, local political bosses and a largely indigenous population that often sympathizes with the traffickers when not working directly for them.

Zeferino Torreblanca Galindo, Guerrero's governor, said in an interview that he backed Calderón's strategy of sending the army into the streets against the traffickers, albeit reluctantly. The municipal and state police were too corrupt and ill-equipped for the task, he said. "If the army doesn't do it, we are left out here on our own."

By the fall of last year, however, the violent consequences of the militarization policy were reverberating through the state. In October, threatening placards known as narco-mantas began to appear along highways. On Dec. 9, the severed head of a Mexican army sergeant was found in a bucket in Chilpancingo, the state capital.

On the morning of Dec. 21, Juan Humberto Tapia, a 38-year-old sergeant, left his house atop a hill in Chilpancingo to walk to work at the headquarters of the 35th Military Zone. The 18-year veteran rarely carried a weapon; he worked as an information technology specialist, repairing the brigade's computers. He planned to retire in two years and open an Internet cafe.

Tapia never made it home that day. The following morning, Chilpancingo awoke to celebrate its Christmas festival. Hours before the parade, Tapia's body, wrapped in green plastic, was discovered by the side of a road entering the city. Next to his headless corpse were the bodies of six other soldiers from the 35th Military Zone, a former police commander and an army recruit. The severed heads were found a mile away, in the parking lot of a Sam's Club.

A profanity-laced message found next to the bodies warned that 10 soldiers would be killed for every drug trafficker killed by the army.

"I don't know exactly what happened to my husband, nor do I want to know," said Tapia's wife, Rebecca Ramirez. "If I start to ask questions — how they kidnapped him, what they did to him — it's just going to hurt more. I'd prefer to remember him for how he was — a person who was good, dedicated, honest."

"This is a consequence of the involvement of the army in the fight against drug trafficking," said Héctor Astudillo Flores, Chilpancingo's mayor. "It's only logical: If you come after me, I'm going to come after you. The army has its specific functions; the constitution spells them out. The constitution doesn't talk about the army involving itself in this type of activity. But the circumstances have aligned themselves in such a way that it has become necessary, and this is one of the consequences."

On a sparkling half-moon bay on Guerrero's coast lies the resort city of Zihuatanejo. Just before dawn Feb. 20, the army swept into the gritty neighborhoods that surround the city's white-sand beaches and luxury hotels and arrested nine men it identified as members of the Beltran Leyva cartel.

The next day, a few minutes before the 9 a.m. shift change, a gray Jeep Cherokee pulled up in front of the Zihuatanejo police station, and someone fired a grenade into the car park. The explosion injured five bystanders and sent a storm of metal fragments into the building's facade.

Four days later, in broad daylight, gunmen attacked four Zihuatanejo policemen with grenades as they patrolled the city's outskirts in a Ford pickup. The officers burned to death in the vehicle. A week later, another Zihuatanejo policeman was shot to death in front of his home; the assailants left a sign that read in part, "I am going to kill every single cop, an eye for an eye a tooth for a tooth." The message also contained a threat to kill the chief of police, "who nobody can get rid of."

"Of course I'm afraid. I'm filled with fear," the 34-year-old police chief, Pablo Rodríguez Román, said at the shrapnel-pocked police station, now protected by sandbags piled five feet high. "But I can't show it to my men. If I do, the entire force will collapse."