The Ken Burns series "The National Parks: America's Best Idea" got me thinking about one of America's worst ideas, the war on drugs. Particularly ill-conceived is the crusade against marijuana.

That bad idea is now threatening the good idea, as Mexican drug cartels -- hampered by a tighter border -- swarm over large swaths of U.S. public land to grow pot. There they dump toxic chemicals, dam streams, clear natural vegetation and leave piles of trash. Marijuana growers building a campfire set off the recent La Brea fire, which scorched 90,000 acres of Santa Barbara County.

Businesses serving tourists warn visitors against armed drug gangs protecting their crops. Last June, for example, hikers in southwest Idaho came upon a marijuana operation with a street value of over $6 million.

Pot farms have been found in, among other places, Redwood National Park in California, North Cascades National Park in Washington state and Pike National Forest in Colorado. An operation in Sequoia National Park was discovered just half a mile from a cave popular with tourists. (Part of the park had to be closed as rangers swooped down from helicopters.)

Federal and state governments spend $8 billion a year enforcing the ban on marijuana -- and they can't even keep the cartels out of Yosemite. The National Park Service, meanwhile, frets about diverting its limited resources from ranger tours to stopping the marijuana growers.

And what purpose does all this spending serve? A new Gallup poll shows that nearly half of all Americans want to legalize marijuana and tax it like alcohol or tobacco. And solid majorities favor permitting medical marijuana, which is now legal in 14 states.

And so the Obama administration's decision to ease up on medical marijuana is not so much leading public opinion toward more enlightened drug policy as following it. Under the new policy, federal agents will not bother users or sellers operating under their state medical marijuana laws.

But while model conservatives (William F. Buckley, Milton Friedman, George P. Schultz) have declared the entire war on drugs a dismal failure, the Republican leadership can't seem to get its mind around ending even the struggle against marijuana. Already widely used, pot doesn't cause the serious health problems associated with cocaine or heroine and often alcohol.

Condemning the new directive, Rep. Lamar Smith, a Texas Republican, said, "If we want to win the war on drugs, federal prosecutors have a responsibility to investigate and prosecute all medical marijuana dispensaries and not just those that are merely fronts for illegal marijuana distribution."

Under the old rules, Drug Enforcement Administration agents raided the backyards of cancer patients permitted by California law to grow pot to ease their discomfort. Millions, however, are still spent ruining the lives of kids caught smoking a joint behind the bleachers.

End the ban on pot, and the drug gangs go away. American farmers find a new business, and government a handsome source of tax revenues. Turning marijuana into a controlled substance could raise $6.2 billion in taxes, according to Harvard economist Jeffrey Miron.

And those who worry about exposing Americans to dangerous drugs would actually sleep better at night. Concern that today's marijuana is much stronger than the pot smoked in the '70s is warranted, but legal products are regulated for potency and purity. Alcoholic beverages became much safer after Prohibition ended. And so would all drugs.

Taking away the illicit profits in pot will make the national parks cleaner and less dangerous. Not many of our problems can be solved by spending fewer taxpayer dollars, but legalizing marijuana is an example. That would make it one of contemporary America's better ideas.

COPYRIGHT 2009 THE PROVIDENCE JOURNAL CO.

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Views expressed in this column are those of the author, not those of Rasmussen Reports.