Now add to that another risk: fueling the “narco wars” in Mexico that have pitted government troops against powerful drug traffickers. At least 22,700 have died in drug-war-related violence in Mexico since 2007, by some estimates. That’s more than eight times the number of Americans killed on September 11, 2001.

“America’s love affair with illegal drugs fuels the major Mexican drug cartels,” former head of the Drug Enforcement Administration Robert C. Bonner said. “[It] allows them to be powerful, corrupt, and to engage in a lot of violence and intimidation in Mexico and elsewhere.” Bonner has also served as a federal judge and Commissioner of U.S. Customs and Border Protection.

Marijuana is often viewed as the most innocuous of illicit substances, and it is the most commonly used illegal drug in the United States. But it is hardly innocent. Many Americans think of cocaine and heroine when they think of the drug wars in Mexico, but by some estimates, marijuana is the most significant source of revenue for the cartels.

“Marijuana represents a good portion of the cash flow of the Mexican cartels, and it represents the most key drug on the southwest border by huge amounts,” Drug Enforcement Administration spokesman Rusty Payne said. “I don’t think people realize that when they increased the demand for marijuana in the United States, oftentimes they directly benefit drug trafficking and the violent cartels in Mexico.”

Harvard economist Jeffrey Miron says that “prohibition pushes the market underground … [resulting in] ancillary negatives,” like the violence and corruption along the border.

Miron points out that not all marijuana in the U.S. is trafficked from Mexico; some is raised domestically. But because of a lack of any product regulation, it is often impossible to know from where – and from whom – a given amount of marijuana comes. Many users inevitably, if unwittingly, help fund the drug wars that haunt the United States’ southern neighbor.

In a July 2010 prepared congressional testimony for a House hearing on international counternarcotics strategy, Adam Isacson, senior policy analyst at the Washington Office on Latin America reported: “Decreased violence in Colombia has been offset by a sharp rise in drug-related homicides in Mexico. Today, Mexico is the center of gravity for groups involved in illegal drug transshipment, which is by far the most profitable link in the drug trafficking chain.”

The tactics and levels of organization of the Mexican cartels also have changed. The Council on Foreign Relations reports that “Violence reached acute levels [in Mexico] in 2006 and has only worsened since then; decapitations became common and cartels began disseminating videos documenting gruesome deaths – ‘narco messages’ – to threaten rival cartels and government officials. While initially the majority of violence was between cartel members, in the past two years, police officers, journalists, and politicians have become frequent targets of drug killings. In May 2008, for instance, Mexico’s acting federal police chief was killed in a drug hit.”

The reports of death and conflict continue. A pregnant American consulate worker and her husband were killed in broad daylight in March of this year, in the violence-ridden border town of Ciudad Juarez. And on July 22, Mexican officials received a tip that led them to several mass graves in the northern part of the country. At least 50 bodies were buried there.

Drug-related conflict could also be creeping over into the United States. The Maricopa County attorney’s office in Phoenix told the New York Times that it received 241 reports of border-related kidnappings or hostage-takings in 2008, compared with just 48 in 2004.

Those numbers pale in comparison to the enormous toll the drug wars have taken south of the Arizona border.

“The Mexican drug cartels pose a national security threat to Mexico in the same way that, at one time, the Colombian drug cartels threatened the Colombian state,” Bonner said.

That, in turn, poses a problem for Mexico’s northern neighbor.

While Bonner said he does not think drug cartel activities pose a substantial national security threat to the U.S. itself, the Mexican government’s battle with the cartels is still of significant concern to the United States.

“It would be untenable [for the U.S.] to have a state on our border that is controlled by narcotraffickers,” he said.

On this side of the border, in 2008, over 40% of American 12th graders had used marijuana at least once, according to the National Institute on Drug Abuse.

Bonner said that while he believes cocaine and methamphetamines are ultimately more profitable for the cartels, “it all adds up.”

When young Americans buy even small amounts of marijuana that is trafficked by the cartels, “they’re contributing to criminal organizations that have engaged in and are engaging in wanton violence in Mexico and elsewhere,” Bonner said.

“They’re aiding and abetting.”

Lawmakers on both sides of the aisle have called the fight a priority, and both Democrats and Republicans have proposed beefed-up security measures on the U.S. side of the border. Most recently, Obama announced plans to send 1,200 National Guard troops to the southwest border.

Congress has appropriated about $1.3 billion in anti-crime and drug funding for Mexico through the Merida Initiative, a multi-year program launched in 2007 that also targets criminal organizations in Central America and the Caribbean.

But the militarization of the fight against drug cartels on the Mexican side of the border, and U.S. support of the effort, has raised red flags in some quarters over escalating violence as well as human rights violations and corruption in the Mexican military and justice system.

At a U.S. Senate subcommittee hearing on drug enforcement and the rule of law May 18, Jose Miguel Vivanco, executive director of the Americas Division of Human Rights Watch, criticized the human rights record of the Mexican military and the lack of accountability for human rights violators. Calderon has relied heavily on the military in his effort to quell the drug cartels.

“Too often local leaders respond to public demands to get tough on crime by condoning abusive practices that not only undermine the rule of law by violating basic rights but also fail to curb crime,” Vivanco said.

In the three years since Calderon launched a military crackdown on drug cartels, about 22,700 people have been killed in drug-related violence.

Beyond the violence, Vivanco’s testimony pointed to alleged abuses by the military, including rape and killings, as well as at least 100 people who claimed to have been arbitrarily detained and then tortured to obtain false confessions since 2009.

Vivanco said that last year Congress should not have given Mexico the 15% of its funding under the Merida legislation that is conditional on fulfillment of human rights requirements. A State Department report to Congress highlighted some issues, including lack of transparency in the military justice system, but found that Mexico had met the four human rights conditions.

Sen. Dick Durbin, D-Ill., chairmen of the subcommittee, was also the sole senator in attendance at the hearing, as his colleagues were occupied with debating Wall Street reform. Durbin called the fight against drug cartels a priority but said the United States has a responsibility to see that its aid does not fuel human rights abuses.

“The military in Mexico in many instances operates with virtual impunity, resulting in limited success in stemming drug violence and human rights abuses that rival and surpass often the corruption of the law enforcement system they were sent to replace,” he said.

Officials from the State Department and Department of Justice testified that Mexico has made significant reforms. The Calderon administration has taken steps to remove suspect law enforcement officials, customs officials and judges and to reform and modernize its judicial system, with U.S. assistance.

David T. Johnson, Assistant Secretary of State for International Narcotics and Law Enforcement Affairs, wrote in his testimony that institutional reforms in Mexico are a work in progress.

“The strategy that the U.S. Government is pursuing with the Government of Mexico is an effective, long-term program, not a temporary ‘quick fix’,” he wrote.

As the drug war continues in Mexico, it’s a debate that will likely be played out many times.

Sens. John McCain and Jon Kyl, both Republicans, announced a proposal that would ramp up many of the measures already in place along their 370-mile portion of the border. That includes deploying an additional 3,000 guardsmen and women and substantially more 24-hour surveillance by unmanned aerial vehicles.

[See the entire plan].

The plan was praised by Arizona law enforcement officials who came to Washington to testify on border security issues, partly in response to the slaying of an Arizona rancher that may have been tied to drug cartels. Pinal County Sheriff Paul Babeu said local authorities are overwhelmed and need help from the federal government, including an increased military presence.

“We are a sovereign nation. We have a right and an obligation to secure our border, and that’s why I’m in full support of Senator McCain and Senator Kyl’s plan of deploying soldiers to the border,”  he said.

But others critiqued the proposal as either going too far or not far enough.

Bradley Schreiber, a former senior adviser at the Department of Homeland Security said the proposal is probably not comprehensive enough to halt the violence on the border. He described it as expanding existing—and largely unsuccessful— measures rather than a new approach to the problem.

“While the senators make some very important additions to what is currently happening, it’s really just a Band-Aid on the dyke ,” according to Schreiber , who now owns consulting firm Homeland Security Solutions, LLC.

Schreiber says the United States needs a ­comprehensive plan for how to deal with the drug trade that includes other governments in the region besides Mexico. Without that, he argued, a clamp-down on the Mexican drug market would simply send the trade and its attendant problems back to the Caribbean, and Central and South America.

At the other end of the spectrum, Isabel Garcia, co-chair of Derechos Humanos, a group that advocates against militarization of the border, blasted the McCain/Kyl proposal as failing to address the root causes of the problem.

“I think the first order of business is to stop the criminalization both of drugs and human beings and end free trade agreements that displace people in Mexico,” she said.

Although McCain and Kyl’s proposal focuses on the U.S. side of the border, U.S. involvement extends into the Mexican interior. Through the three-year Merida Initiative, the U.S. has committed $1.4 billion in anti-drug aid to the region, with the bulk of the money going to Mexico.

At a Senate Armed Services Committee hearing last week McCain spoke of the danger to U.S. national security if the Mexican government were to fall to the drug cartels. He reiterated the point in an interview on Monday, following a press conference on border security.

For now, however, it is Washington’s ally to the south that is in dire danger, McCain and others say.

“The president of Mexico has stated that they’re an existential threat to the government of Mexico – that’s what the president said, so I certainly take him at his word,’’ McCain said. “When you have 22,000 of your citizens who have been murdered by the drug cartels, it’s a threat to the existence of the government.”