Well, it could have been true. That's what Senator Hillary Clinton had to say after finding out that five Pakistani men did not actually sneak into the US through Canada so they could blow up New York on New Year's Eve. Because they were never in the US at all, and they weren't terrorists, and the whole thing was dreamt up by a man who forges passports for a living.
At the height of the search for the professional liar's imaginary non-terrorists, Clinton had blamed Canada and its "unpatrolled, unsupervised" border. But even when the hoax came to light, Clinton didn't rescind the accusation. Because the Canadian border is so porous, she reasoned, "this hoax seemed all too believable".

It was, in other words, a useful hoax, helping US citizens to see how unsafe they really are. And that is useful, especially if you are among the growing number of free-market economists, politicians and military strategists pushing for the creation of "Fortress Nafta", a continental security perimeter stretching from Mexico's southern border to Canada's northern one.

A fortress continent is a bloc of nations that joins forces to extract favourable trade terms from other countries, while patrolling their shared external borders to keep people from those countries out. But if a continent is serious about being a fortress, it also has to invite one or two poor countries within its walls, because somebody has to do the dirty work and heavy lifting. It's a model being pioneered in Europe, where the European Union is currently expanding to include 10 poor eastern bloc countries, at the same time that it uses increasingly aggressive security methods to deny entry to immigrants from even poorer countries, like Iraq and Nigeria.

It took the events of September 11 for North America to get serious about building a fortress continent of its own. After the attacks, it wasn't an option for the US simply to build higher walls at the Canadian and Mexican borders; in the Nafta era, the business community wouldn't stand for it. General Motors claims that for every minute its fleet of trucks is delayed at the US-Canadian border, it loses about $650,000.

On the other US border, dozens of industries, from agriculture to construction, are reliant on "illegal" Mexican workers - a fact not lost on George Bush, who knows that, after oil, immigrant labour is the fuel driving the southwest economy. If he suddenly cut off the flow, the business sector would rebel. So what's a wildly pro-business, security-obsessed government to do?

Easy. Move the border. Turn the Mexican and Canadian borders into glorified checkpoints and seal off the entire continent, from Guatemala to the Arctic Circle. Bush officials don't talk much about the continental fortress, preferring terms like "North American area of mutual confidence". But a US-run security perimeter is precisely what is being built.

In the past year, Washington has pressured Canada and Mexico to harmonise their refugee, immigration and visa laws with US policies. And in July 2001, Mexico's president, Vincente Fox, introduced Plan Sur, a massive security operation on Mexico's southern frontier that immigration experts refer to as "the southern migration" of the US border. Under Plan Sur, the Mexican government has deported hundreds of thousands of mainly central Americans on their way to the US, with the US itself providing much of the funding. In one bizarre incident last year, Mexican guards caught a group of Indian refugees on their way to the US, bussed them to a squalid refugee detention centre in Guatemala, and Washington paid the cost ($8.50 a day per detainee).

Fox had hoped to be rewarded for policing the undeclared US southern border, and he used to have reason for optimism. As recently as September 6 2001, Bush was pledging to "normalise" the status of the roughly 4.5 million Mexicans living illegally in the US. After September 11, however, the status of these workers became even more precarious.

This points to another truth about fortress continents: being on the inside may be better than being locked out, but it's no guarantee of equal status. Washington is constructing a kind of three-tiered fortress in which the US rules by decree, Canada and Mexico serve as guards, and Mexican workers are banished to the continental equivalent of the servants' quarters.

Inside Fortress Europe, France and Germany are the nobility and lesser powers such as Spain and Portugal are the sentinels. Poland, Bulgaria, Hungary and the Czech Republic are the postmodern serfs, providing the low-wage factories where clothes, electronics and cars are produced for 20-25% of the cost to make them in western Europe.

The huge greenhouses of southern Spain, meanwhile, have stopped hiring Moroccans to pick the strawberries. They are giving the jobs instead to white-skinned Poles and Romanians, while speedboats equipped with infra-red sensors patrol the coastline, intercepting ships of north Africans. Increasingly, the EU is making "repatriation agreements" an explicit condition of new trade deals. We'll take your products, the Europeans say to South America and Africa, as long as we can send your people back.

What we are seeing is the emergence of a genuinely new new world order, one far more Darwinian than the first, second and third world. The new divisions are between fortress continents and locked-out continents. For locked-out continents, even their cheap labour isn't needed, and their countries are left to beg outside the gates for a half-decent price for wheat and bananas.

Inside the fortress continents, a new social hierarchy has been engineered to reconcile the seemingly contradictory political priorities of the post-September 11 era. How do you have airtight borders and still access cheap labour? How do you expand for trade, and still pander to the anti-immigrant vote? How do you stay open to business and closed to people? Easy. First you expand the perimeter. Then you lock down.