BILLINGS, Mont. - If Montana and Wyoming were independent nations, troops would be lined up at the border waiting for an order to fire.

At stake are the troubled waters of the Tongue and Powder rivers that rise in the Big Horn Mountains of Wyoming and flow across the state line into reservoirs and irrigated fields in Montana.

During drought years, and there seem to be more and more of them, the Yellowstone River tributaries can't provide enough water to meet demands on either side of the line.

In a scenario likely to become common as the West gets warmer and precipitation grows less reliable, Montana argues that Wyoming is taking more than its share of a scarce resource.

A lawsuit submitted by Montana in January asks the U.S. Supreme Court to divide the waters by interpreting disputed provisions of the 56-year-old Yellowstone River Compact, an agreement approved all those years ago to avoid just such a conflict.

But in 1950, who anticipated 23,000 coal-bed methane wells that disgorge millions of gallons of groundwater? Who foresaw sprinkler systems so efficient that less than 10 percent of the water used to irrigate crops finds its way back to the rivers as they head toward Montana? And what did we know five decades ago about global warming that would raise the average temperature in the West 2 degrees by 2006 and melt snowpack two to three weeks earlier?

Montana sees the lawsuit as a matter of economic necessity. Wyoming sees it as a frivolous, premature and costly legal battle with little basis in fact.

Wars have been fought for less.

In its lawsuit, Montana Justice Department attorneys describe the conflict as "a dispute between states of such seriousness that it would amount to 'casus belli' if the states were fully sovereign. Casus belli is defined as an act or circumstance that provokes or justifies war."

The compact, which was endorsed by Congress, constitutes a treaty, Montana argues.

"Violation of a treaty is one of the classic occurrences giving rise to war," the lawsuit said.

The battle will be fought in courtrooms, and the weapons will be thousands of pages of legal documents. And it's likely to stretch on longer than the wars in Vietnam or Iraq. Some interstate water disputes remain unsettled for decades.

The compact

During the Dust Bowl days of the Great Depression of the 1930s, the importance of managing water became apparent to a country bent on prevention of further agricultural catastrophe. Federal water projects were being considered by the Bureau of Reclamation in the Yellowstone Basin, but the government wanted Montana and Wyoming to reach an agreement on interstate allocation of the waters first.

After 20 years of negotiation, the states agreed on the current Yellowstone River Compact on Dec. 8, 1950. Legislatures in both states and Congress approved the compact in 1951.

The compact runs just eight pages in the Montana Code Annotated, including two pages listing signatories and commissioners. It's a pretty basic document aimed largely at protecting the existing rights of water users in both states and preparing the way for federal water projects, which, for the most part, never happened. Only Yellowtail Dam on the Big Horn River was built.

The compact is administered by a commission composed of one representative from each state selected by its governor plus a representative of the U.S. Geological Survey. Administrative disagreements can be settled by a majority vote of the commission. However, as a matter of policy, the federal government's representative never votes to break a tie.

One critical omission in the compact, which is at the heart of today's controversy, is that those pre-1950s water rights grandfathered into the agreement were not quantified. The compact never states how much water Montana and Wyoming users with pre-1950s rights were entitled to divert.

Now Art Hayes, president of the Tongue River Water Users Association, has to make choices he never faced before.

"In really short water years, you have to pick and choose what field is going to get water," he said. "A field not in top production might not get much water. It's really tough. You get best production out of young stands of alfalfa. When you put in new alfalfa, it doesn't produce much at first, but you have to keep it fairly moist."

'A tragedy'

From Wyoming's perspective, the drought is the problem.

"We've had some drought years, and we're all suffering, but show us how we've violated the compact. Never in their briefing with the court have they showed that," Wyoming Attorney General Pat Crank said of Montana's lawsuit. "Every year, there's a lot of water that flows that's never used and never stored. Under the way you calculate post-1950 usage, Wyoming is using just a small percentage of what it's entitled to with regard to our post-1950 usage."

He also said the proper vehicle for resolving the dispute is the Yellowstone River Compact Commission.

"From my perspective, Montana jumped the gun. They never gave that forum the chance to work," Crank said.

"For a whole bunch of years since 1950, we've kind of worked through these water problems that have occurred during dry years and years of drought," Crank added. "But now, Montana came to the commission with some very strained legal interpretation and put a motion in front of the commission. And when that died for lack of a second, they ran and filed a lawsuit in the U.S. Supreme Court."

Crank said Wyoming has operated "well within the confines of the compact" and that Montana's complaint "is hedged in large part with language like 'may have potential damage.'"

Crank noted that it would cost taxpayers in Montana and Wyoming millions of dollars to litigate the case, "and that's a tragedy for both states."

Montana has been concerned about diminishing waters of the Tongue and Powder rivers since the dry years of the 1980s, according to Mary Sexton, director of Montana's Department of Natural Resources and Conservation and Montana's representative on the Yellowstone Compact Commission.

"The '90s were a little wetter, and it was not quite as tense," she said. "Then beginning in 2000, the drought really exacerbated the situation."

"It's a heated issue all over the West, and it's likely to stay that way," Sexton said.