The mounting drug-gang violence along the Mexican border spilling into Tucson, Atlanta and other American cities–the kidnappings, beheadings, torture and street massacres–is as unsurprising as it is ironic. It’s also a loud call for a fundamental review of this nation’s so-called “war on drugs.”

It’s unsurprising because Mexican President Felipe Calderón’s attempt to assert his authority and please his American neighbors by cracking down on the drug cartels is jeopardizing established supply routes and the cozy arrangements with Mexican drug cops and public officials that in effect stabilized the trade. Now the gangs, which are also seizing a growing segment of the migrant-smuggling business, are at war not just with Mexico’s federal cops, the army and the shrinking number of honest prosecutors and judges who try to confront them but with one another for a share of that destabilized market.

It’s ironic because Americans, as everybody knows, buy and consume most of the marijuana, cocaine and methamphetamines that make up the lion’s share of the cartels’ business. Also, some 90 percent of the guns used by drug gangs, including a growing number of military-type assault weapons, come from American gun dealers, most in the border states. And it’s American politicians who’ve been most active in putting the heat on Calderón to stop the drugs. But perhaps most ironic of all, it’s the federal prohibition of marijuana, first enacted in large measure because of its association with Mexicans, that created the multibillion-dollar market in which the drug gangs thrive.

Which is to say that the real cause of the drug violence now drawing anxious attention from Congress, the Obama administration and worried state and local law enforcement officials lies on this side of the border.

What’s new is that for the first time Washington is beginning to pay attention. At a Senate judiciary subcommittee hearing in March on the violence at the border there was widespread agreement that America deserves a large portion of the blame. At almost the same time, Secretary of State Hillary Clinton acknowledged a shared responsibility. “We know very well,” she said during a visit to Mexico, “that the drug traffickers are motivated by the demand for illegal drugs in the United States, that they are armed by the transport of weapons from the United States to Mexico.” When President Obama met Calderón recently, drug violence joined immigration at the top of the agenda. The two issues are linked in more ways than even Obama probably knows.

Historians disagree on who or what was the largest influence in persuading Congress to pass the Marijuana Tax Act in 1937, which in effect outlawed marijuana and criminalized its production and sale. Was it newspaper baron William Randolph Hearst, or was it Harry Anslinger, who as head of the Federal Bureau of Narcotics was the nation’s first drug czar?

Hearst had it in for Mexicans ever since Pancho Villa confiscated some 800,000 acres of his timberland during the Mexican Revolution. But the campaign against “reefer madness” and all the horror stories associated with this “sex crazing drug menace”–killers maddened by marijuana, children seduced and destroyed by dope, field hands so blissed out under the influence that they couldn’t do their work–also made great circulation-boosters.

With the end of Prohibition in 1933 and Congressional questions about Narcotics Bureau funding, Anslinger, trying to protect and enlarge his empire, campaigned hard to add marijuana to the list of controlled substances. Busily cranking out testimony, press handouts and articles contending that pot is “as dangerous as a coiled rattlesnake,” he became the loudest voice in Washington for marijuana prohibition.

Given the law enforcement disaster alcohol prohibition had become, Congress should have been particularly wary of heading down that road again. But the record of the 1937 House hearings makes clear that Congress never got any sound medical evidence on the dangers of cannabis, and didn’t ask for it. When they voted for the law, most members had no idea what marijuana was. But they had heard Anslinger tell them that “its use frequently leads to insanity.”

In the context of today’s conflicts with Mexico, there may be an even larger irony. For white supremacists like C.M. Goethe, a California millionaire and eugenicist who campaigned relentlessly to have Mexico added to the list of countries from which immigration was severely restricted, linking marijuana to Mexicans proved their racial inferiority and unfitness to become Americans. Marijuana was bad because Mexicans used it; Mexicans were bad because they used marijuana.

The passage of the Marijuana Tax Act effectively locked pot prohibition into federal law. Even in the past decade, as voters in a growing number of states legalized it for medical uses and liberalized other drug laws, federal agents raided medical marijuana dispensaries in California and other states where it was lawful. Barry McCaffrey, President Clinton’s drug czar, tried to revoke the narcotics-prescribing licenses of doctors who recommended medical marijuana to their patients, but he was stopped in the courts. In the meantime, growing numbers of Americans, future presidents among them, casually used and continue to use marijuana.

The reform of the draconian Rockefeller drug laws in New York was another illustration of Americans’ growing willingness not just to liberalize marijuana use but a range of other drug policies away from prison and punishment toward a greater emphasis on treatment and what drug law reformers call “harm reduction.” Given those changing attitudes, driven at least in part by the huge number of voters who have themselves used illegal drugs, the violence at the border ought to be another powerful reason to re-examine the nation’s costly, ineffective and ultimately destructive drug policies. If marijuana were treated like alcohol or tobacco, the largest revenue sources of the Mexican gangs would dry up.

Something similar applies to Congress’s continuing tolerance of assault weapons. At the Senate hearings in March, Senators Dick Durbin of Illinois and Dianne Feinstein of California made clear that they understood the role of American arms merchants in supplying guns.

But the immediate prospects for restoration of the federal ban on assault weapons that Feinstein had pushed through Congress and whose extension had been throttled by the NRA in 2004 aren’t much better than those for drug law reform. The right to bear arms is constitutionally protected, said Senator Jeff Sessions of Alabama at the March hearings. If the Mexican gangs didn’t get their guns from the United States, they’d get them elsewhere.

Attorney General Eric Holder says the administration will end federal raids on providers of medical marijuana in the thirteen states that allow it, provided that they violate no state laws. And at least one bill has been filed, by Senator Jim Webb of Virginia, creating a commission to review drug policies. But neither Obama nor most others in Washington seem prepared to begin any sort of comprehensive review of the marijuana laws, much less address drug law reform generally.

As for the drug-gang violence, Obama said in March that he wasn’t yet ready to send the National Guard and further militarize the border, as a lot of close-the-border Republicans and talk-radio right-wingers were demanding. So far, he’s only sending more agents and hardware to try to check the southward flow of guns and cash and the northward flow of drugs. But that’s less than half a solution, which requires the nation, from the president down, to address comprehensive immigration reform, drug law reform and gun control. All are major elements in the border violence; all are crucial if we’re going to stop the blowback. We’ve hardly started on any of them.