It was a procession straight out of a children's book: A Montana sheriff's SUV and a federal law enforcement truck driving down U.S. Highway 287 toward Yellowstone National Park, bright lights flashing, followed by five shaggy bison. Make way for buffalo! Behind them were more government trucks, several guys on ATVs and seven horseback riders. Overhead, a helicopter thumped.

They kept it up for miles; the bison ran so hard their tongues hung down.

But not everyone was welcome to join the parade. A sheriff's deputy warned a vanload of journalists to stay clear or be arrested.

"Arrested for what?" asked a ticked-off Los Angeles Times reporter.

"Obstructing."

"Obstructing what?"

Obstructing the Interagency Bison Management Plan.

This plan was made 10 years ago through negotiations between the National Park Service and other powers, including the Montana Department of Livestock. It's designed to prevent the spread of a bacteria-borne disease, brucellosis, that can cause cattle and bison to abort their calves. And it is a great illustration of Western environmental gridlock.

Ranchers -- always a formidable special interest -- worry that bison might spread brucellosis to cattle. Therefore, Yellowstone is allowed to have no more than 3,000 bison, more or less, and those can't stray. So every spring, when hundreds of them naturally venture out of the park seeking lower-elevation grass and good calving grounds, government agents "haze" them back in. The government also rounds up bison to test them for brucellosis, and slaughters any that test positive. Some years, more than 1,500 are killed.

Conservationists want to stop all this and let the bison wander beyond the park's arbitrary boundaries. That's why the Natural Resources Defense Council and the Buffalo Field Campaign invited journalists on the May field trip that included the hazing on Highway 287. "Bison were the paramount iconic species of the West," Keith Aune, a Wildlife Conservation Society biologist, explained.

Ironically, cattle spread brucellosis to Yellowstone's wildlife in the first place; ranchers eventually culled the infected cows, but even though the government has spent more than $20 million on the bison plan, the disease persists in bison and elk because wildlife diseases are nearly impossible to eradicate in the wild. And the government isn't investing much in developing a better vaccine against brucellosis.

Moreover, there are only 20,500 wild plains bison left in North America, in 62 "conservation herds" run by agencies and conservationists -- a tiny fraction of the 25 million to 60 million that once roamed the Great Plains, Aune said. That decline is masked by the half-million bison raised for meat on private ranches. So some agencies and conservationists are reintroducing wild bison to places like central Montana, even as Yellowstone's bison -- the largest wild herd -- are being slaughtered.

The absurdities don't end there: Cattle in all three states bordering Yellowstone have been infected since the plan was imposed. But the suspected culprits are elk, not bison.

Native Americans, despite their cultural ties to bison, are reluctant to accept Yellowstone's surplus on some reservations because their ranchers also fear brucellosis. But conservation-oriented billionaire Ted Turner has just accepted 87 Yellowstone bison on one of his Montana ranches, on condition that he gets to keep many of their offspring. In response, some conservationists have filed a lawsuit charging that he's privatizing public wildlife.

Other pro-bison forces including the National Wildlife Federation have retired cattle-grazing permits on Horse Butte, a key area next to the park, and some locals say the bison are welcome to hang out in their yards -- ignoring the threat of bodily injury. (Tourists in the park are occasionally trampled and gored.) But the federal plan still orders that bison be hazed off Horse Butte.

Errol Rice, a fifth-generation rancher who's executive vice president of the Montana Stockgrowers Association, explained his side's thinking: No matter where the line is drawn, sooner or later the bison population will expand to cross it. When cattle are infected, ranchers must slaughter whole herds. "It can bring a ranch family to its knees," Rice said.

The conservationists say the risk is manageable and worth taking. But ranchers want zero risk or full compensation for their losses.

Along Highway 287, the hazing forces eventually gathered about 80 bison and shooed them to the intersection with Highway 191, where they turned right heading for the park. One conservationist used his iPhone to Twitter from the front lines: "I am watching bison being hazed ... Wow!" Clearly, the problem is not yet solved.