In Michael Moore’s 1995 Canadian Bacon, an American president decides to boost his re-election prospects by going to war against Canada.

Canadians were not particularly amused, but neither were they upset. This was a fictional invasion. Besides, it only sought to capture their government.

Now looms a U.S. invasion Canadians take more seriously. This one is real, and its target is more tangible -- their water. They think we’re coming after it. They’re right.

It isn’t that the water wars are the talk of the nation; they were rarely mentioned in the recent federal election campaign. But the dispute bobs beneath the surface, a regular topic of conversation among the political elites. From the left, the Council of Canadians calls for a national water policy that would prevent “bulk water exports and diversions.” From the right, former Albert Premier Peter Lougheed predicted that, “the United States will be coming after our fresh water aggressively within three to five years.”

So far as is known, the Department of National Defence has not organized a task force to determine where to erect barricades. No one expects the U.S. Marines to land on Cape Breton or the Army’s 82nd Airborne to drop into Vancouver. This will be drip and drain, not shock and awe. What Canadians fear is not incursion, but diversion.

President Bush hadn’t been in office eight months when he first mused in public about bringing Canadian water to the American Southwest. And no sooner had 2006 begun than former U.S. ambassador Paul Cellucci told a Canadian audience that their resistance to selling water south was “odd.”

If so, it is oddness shared by 69 percent of Canadians, according to a 2002 poll by the Centre for Research and Information on Canada. But then, Cellucci, probably the least popular ambassador the United States has ever sent to Ottawa, has been politically tone deaf for some time.

Still, he may have a point. Canada has 20 percent of all the world’s fresh water, to slake the thirsts and irrigate the crops of only 0.5 percent of the world’s population. With the United Nations estimating that almost two-thirds of everybody, or almost 5.5 billion people, will face chronic water shortages by 2050, you’d think the Canadians could ship a few gallons elsewhere, especially if they get paid for it.

After all, if water is going to become as scarce as the United Nations suggests, the forecasters who predict that water will be “the next oil” -- the resource that nations go to war over -- could be right. And keeping it from all those thirsty billions would become morally as well as geo-politically untenable.

Not that what Bush, Cellucci et al. have in mind is providing succor to the parched patches of Africa or Asia. What they’re talking about is making sure that the suburbanization of greater Las Vegas, Phoenix, and smaller metropolises in the Southwest is not impeded by lack of H20.

It’s a long way from Las Vegas to Alberta, but water entrepreneurs do not think small. Some 20 years ago, the North American Water and Power Alliance proposed to dam most of the rivers in British Columbia and divert the water into the United States and Mexico through a half-billion-dollar network of dams and canals. Right now, a California company has a plan to put water from western Canadian rivers into plastic bags bigger than the Goodyear blimp so it can be towed south by ship. And in 1998, Ontario said an outfit called the Nova Group could export millions of liters of Lake Superior water by tanker to Asia.

Politically speaking, that one opened the floodgates. You don’t mess with the Great Lakes without arousing passions, and opposition, on both sides of the border. That plan was swamped, and both federal governments agreed to consider stronger protection for the lakes.

Immense though they are, the Great Lakes may not be big enough to survive their new world, one complicated by global warming and the North American Free Trade Agreement. According to an August 2004, report by the International Joint Commission, one of the bi-national bodies established to govern and protect the Great Lakes, most climate change models predict lower lake levels as the earth warms. And the same report appears to acknowledge that once a body of water has become “a commercial good or saleable commodity,” any effort to protect it could fall afoul of NAFTA. The message seems to be that if you want to protect any of the lakes, or perhaps any bays or inlets thereof, pass the law before some company starts selling the water.

Late last year, the governors of the eight Great Lakes states and the premiers of Quebec and Ontario agreed on changes to the annex to the 1985 Great Lakes Charter. If ratified by Congress and the Canadian Parliament, the changes would make it harder to divert water from the lakes.

But not hard enough, according to Susan Howatt, the national water campaigner for the Council of Canadians. “It’s full of loopholes," she said of the 1985 charter, including one for bottled drinking water companies -- if the water goes into containers of less than 20 liters, it’s not considered a diversion.

Not all Canadians oppose water commerce. In the Maritime Provinces and in Western Canada, some businessmen, free-marketish scholars and a few public officials think it would be a great idea. But they acknowledge that theirs is a minority view. “Canadians tend to get rattled at just the thought of selling water to the U.S,” wrote Daniel Klymchuk, a real-estate developer in Western Canada. “It’s an irrational fear that will take time to dissipate.”

Beyond the specifics of this dispute is a question of political philosophy which is likely to become more salient in the years ahead, and which poses a special dilemma to those who lean politically left: should water be considered a common natural heritage or a sellable commodity?

By temperament, liberals might be drawn to the view of Canadian writer Marq de Villiers that “water is not ‘ours,’ or ‘theirs,’ but the planet’s.” And there is little doubt that massive diversions could prove environmentally disastrous.

But then there will be all those thirsty people on their dried-out land, and in 1997 the United Nations concluded that the best -- perhaps the only -- way to get water to them was through a system of international markets and trade.

Closer to home, Americans and Canadians could alleviate the problem by wasting less water. We are the two most water-profligate nations in the affluent world. We might, for instance, stop irrigating (often at public expense) fields that produce crops already in surplus.

Or we could pursue what Susan Howatt called “the soft path” to water conservation, which would mean limiting the population of an area to the number of people its natural water supply could support.

Well, she’s a foreigner, so perhaps can be forgiven for such subversive talk. Had that been our policy, Southern California would be home to hundreds of thousands, not tens of millions. Someone should tell Howatt that, at least on this side of the border, science and logic are all very well in their place, but real-estate development rules.

Jon Margolis, the former national political correspondent for the Chicago Tribune, is the author of The Last Innocent Year: America in 1964.