BLAINE, Wash. - The invisible line that divides Canada and the United States runs along a shallow ditch just beyond Shirley-Ann Leu's back yard - so close she could cross the border in a single hop.

At 72, Shirley-Ann, a retired hairdresser, shows no such inclination. But some in her care - namely 11 Pomeranians, two toy poodles and a young neighbor girl she baby-sits - appear to her all too eager to jump the ditch and roam wild in Canada.

To prevent this, Shirley-Ann and her husband, Herbert, 69, a retired electrician, built a 4-foot-high concrete wall. They saw it as a perfect solution. The wall would enclose their wards and also keep their sloped back yard from crumbling into the ditch.

Little did they know it would instigate a bona fide border conflict.

The Leus now find themselves in a legal fight against the U.S. government, which has the support of the Canadian government. The outcome will determine whether the wall stays, which party will pay if it has to be removed, and to what extent border authorities can control development on private property.

Because the Leus' wall is part of a larger project that includes a driveway, walking paths and patios - the entire outdoor portion of their quarter-acre lot - the couple has stopped all construction until the case is resolved. The work involves heavy machinery, and the Leus don't want to pay the extra cost of doing the project piecemeal.

In her back yard, Shirley-Ann stands amid mounds of lumber and rebar, looking as if she's about to cry.

"We were supposed to retire and spend our days in peace," she says. "Instead…." She raises her hands above her head and mimes pulling her hair out.

Life for the couple has been complicated since they moved from Hawaii a year and a half ago to this rural border town 110 miles north of Seattle. The wall dispute, involving an obscure agency and a little-known treaty, is only the most aggravating part of the strange reality of living next to an international boundary.

How can she explain it?

Shirley-Ann pads around the wall to the edge of her property and places a foot in the ditch, her fuzzy black house slipper set daintily against the dirt.

"That's Canada," she says, her words weighted with the notion that her foot was now subject to the rules of a different nation.

Running parallel with the ditch is a two-lane road, Zero Avenue, part of the Canadian municipality of Surrey. A handful of homes dot a rolling landscape of fields and pastures. Cars zip past at highway speed although the signed limit is 50 kilometers (31 mph).

Every few hours on Zero Avenue, a Royal Canadian Mounted Police vehicle drives past. At the front of the Leus' house, on the American side, the U.S. Border Patrol makes regular passes along West 99th Street. The Leus say it took a while to get used to the patrol cars passing day and night on both sides of their house.

Underground sensors hidden on the edges of Zero Avenue detect all kinds of movement. One neighbor on the American side, Bob Boulet, says every time he mowed his backyard last year, he set off a sensor and a Border Patrol helicopter came whizzing by within minutes. Boulet says he asked the Border Patrol to mow the lawn for him so he wouldn't bother them anymore. The agency didn't find the request amusing.

Between Boulet's house and Shirley-Ann's, a surveillance camera swivels atop a 40-foot tower, watching and recording the goings-on in a 360-degree panorama.

All of this, for Shirley-Ann, translates to one thing: "Don't even think about crossing the road," she says, moving her foot back to the United States. "Try it. They'll come after you."

Which is why, in her mind, it would be disastrous if one of her dogs or the little neighbor girl ever jumped the ditch and crossed over. First, they could be run over by a car. But if they made it, how would she get them back?

The quickest legal way to retrieve them would be to drive west two miles into town, get in line at the truck crossing at Highway 543, show her documents, and drive back east onto Zero Avenue. It could take 10 minutes to get there and as long as three hours to get back, depending on traffic.

Or she could take her chances and run across the street. If she was caught, she could face a range of penalties. Crossing the border without passing through an official point of entry breaks the law on both sides.

On the U.S. side, according to Border Patrol agent Joseph Giuliano, deputy chief in the Blaine office, a border jumper could face a $150 citation or up to a $5,000 fine and six months in jail.

A former fishing and logging town, Blaine, population about 4,000, is best known as the northern terminus of Interstate 5 and the third-busiest checkpoint on the U.S.-Canada border, screening about 7 million passenger cars a year.

East of the border station, on the outskirts of town, a little neighborhood surrounded by woods sits in relative isolation.

Besides the rural feel, the neighborhood's biggest appeal is hinted at by the name of one of its main streets: Canada View Drive. On this road, which becomes West 99th Street, the houses along the ditch look out at British Columbia's lush Fraser River Valley and beyond to the Coast Mountains.

It was this view that first hooked Shirley-Ann and Herbert. Their stucco ranch-style house was built with large picture windows facing Canada.

"Look at that," says Shirley-Ann, sitting in a bay window of her living room.

Herbert, an exceedingly genial man with gray at the temples, sits nearby, smiling, letting his wife do the talking and occasionally chiming in with a "Yes, yes. That's right."

Their story in sum: lived in Hawaii 48 years, married 42 years, no children, recently retired. The couple found their fixed income didn't go far in Honolulu. "Some days we were living on pork and beans and toast," Shirley-Ann says. Herbert smiles in affirmation.

In 2005, the couple sold their home in Hawaii and moved here, where land was cheap and they could be closer to Shirley-Ann's relatives in Canada.

They hired a contractor to build the wall. Workers cleared the ground in August and poured the concrete in November. The wall stretched 85 feet and cost $15,000. In January came the knock at the front door.

The man identified himself as a field engineer for the International Boundary Commission. Shirley-Ann vividly recalls the conversation:

"He says to me, `Your wall violates the treaty."'

"I have no idea what you're talking about," she responded.

"The Treaty of 1925," the man says, handing her a brochure. "Someone will be contacting you." Before the man left, Shirley-Ann asked a question she has asked numerous times since. "Who are you again?"

She and Herbert had never heard of the International Boundary Commission. Neither had the city of Blaine, which had approved the wall and issued the permits. "I was surprised, too," says Blaine city manager Gary Tomsic, referring to the IBC. The treaty was news to him.

The official name is the Treaty Between Canada and the United States of America To Define More Accurately and to Complete the International Boundary Between the Two Countries.

The document, signed by then-U.S. Secretary of State Charles Evans Hughes and Canadian Minister of Justice Ernest Lapointe, established the IBC as a binational agency with one commissioner in each country, and charged the agency with maintaining an "effective" boundary. This meant, in part, maintaining a clear 20-foot-wide "boundary vista," or swath - 10 feet on each side - along the entire border.

In a hand-delivered letter to the Leus in February, the IBC told the couple that the wall, although inside their property line, encroached 30 inches into the boundary vista and would have to be torn down or moved.

"If the wall is not removed and/or relocated … then the commission may itself cause the wall to be removed and the expenses for the removal will be invoiced by you," wrote the U.S. commissioner for the IBC, Dennis Schornack.

In April, the Leus filed the first lawsuit ever brought against the agency.

The Leus' lawyer, Brian Hodges, says the IBC has no authority to condemn property or regulate development on private lots without providing adequate notice to owners and local governments. The fact that Blaine officials knew nothing about the boundary-vista setback, Hodges argues, indicates the agency had not done a satisfactory job of notifying the public.

In a phone interview from his Washington, D.C., office, Schornack said he could not comment on the Leus' case except to say property owners close to the border have "a common-sense obligation" to inquire about federal restrictions.

Schornack's half of the agency (the other half is in Ottawa) has an annual budget of $1.4 million. "Budget dust," Schornack says.

The amount is barely enough to accomplish the agency's primary mission, which is to maintain a clearly visible boundary. This means hiring work crews to go into remote areas with chain saws and machetes. Some sections in Alaska, Washington, Idaho and Montana have not been cleared in decades.

In a sense, the IBC is a victim of the historically good relations between Canada and the United States, said deputy commissioner Kyle Hipsley. Because "we've always been friends with Canada," Hipsley says, the work of the agency isn't seen as an urgent funding priority in Congress.

Schornack and Hipsley say they hope Congress will realize the importance of their work: The agency has asked to double its budget for 2008.

In Seattle, the U.S. attorney's office is preparing a response on behalf of the IBC. The deadline is in June. The Leus continue to stare out at the construction site that would be their back yard.

Shirley-Ann, unable to set her dogs loose or plant flowers in the garden, pores over documents related to the case. Stacks of paper litter the floor beneath her chair in the bay window. Occasionally she glances out at the view, as if trying to remember something. Every once in a while she issues a statement directed at the world.

"Retire in peace. Ha," she says.

"Yes," says Herbert, still smiling. "That's right."