THERE was a time when Canadians boasted that the line separating Canada from the United States, stretching for 8,900 kilometres (5,500 miles), formed the world's longest undefended border. Security measures taken by the American administration in the wake of the terrorist attacks of September 11th 2001 put an end to that. Talk of the border these days tends to focus on how much more difficult it has become for goods or people to cross it. While much of the world has the same concern, the problem is particularly acute in Canada because the country's southern neighbour remains its largest trading partner by far, with C$1.6 billion ($1.6 billion) worth of goods crossing the border each day.

In the last nine years, Canadian governments have tried to keep the border as open as possible for trade while at the same time allaying American security fears with a series of deals. It hasn't worked. Exporters continue to complain about a thickening of the border, which has helped reduce goods shipments to the United States from 87% of the total in 2001 to 73% in 2009 (though the emergence of new trading partners in the developing world and a lengthy recession south of the border also played a part).

So in some ways it was not a surprise when Stephen Harper, the Canada's conservative prime minister, confirmed in one of his year-end interviews that Canada was negotiating a new security and economic deal with the Americans. Rumours that something called a perimeter security arrangement was in the works had been circulating for weeks. One media outlet even got its hands on the communications strategy the government would use to sell the deal to Canadians sceptical of giving away sovereignty in exchange for market access.

In the interview Mr Harper was deliberately vague about the contents of the deal and whether it would be ready for Barack Obama to sign during a planned visit to Ottawa early in 2011. His reticence may have had something to do with the state of the talks. But it was more likely caused by a desire to keep opponents in the dark as long as possible. For there will be formidable opposition, both from within Canada and from without.

This is not the first time Canada has been down this road. There was talk of the desirability of a perimeter security arrangement, dubbed Fortress North America, shortly after the 2001 attacks. The Security and Prosperity Partnership, which was launched with much fanfare by the three North American leaders in 2005 but has since withered away, was thought to be a step in this direction. But no one was completely clear on what perimeter security entailed. Analysts reckoned it could be as simple as using technology to make crossing the border more efficient, something that was put in place after the two countries signed the Smart Border Declaration in December 2001. It could go beyond that to moving customs clearances away from the physical border and allowing each country's customs officials to do clearances for both. Its ultimate expression would be to harmonise immigration, customs and national security policies, doing away with border inspections altogether and establishing a perimeter zone around Canada and the US.

This European model would be unpalatable to those Canadians who fear that they would have to adopt American policies. One strategist for the opposition Liberal Party has already proposed that they run ads similar to those used during the 1988 national election campaign when the main issue was whether to adopt a free trade agreement with the US. The ads featured an American trade negotiator erasing the border with Canada.

It is not just Canadian nationalists, however, that Mr Harper has to worry about. Mexico, a partner in the North American Free Trade Agreement (NAFTA), has not been included in the talks despite the fact that the idea of a North American security area was initially proposed as NAFTA-plus by Vicente Fox, the then Mexican president. Mr Harper reckons, with some justification, that the problems at the the United States's northern frontier are different to those at its border with Mexico. Less plausibly, he also suggests that Canada has a special relationship with the United States that precludes Mexico.

It is not clear where the Americans stand on all of this. Congress passed a law in 2002 calling on the administration to conduct a feasibility study of something called the North American National Security Perimeter Program, which was meant to enhance the security of the three NAFTA partners. It looks likely that Mr Harper is trying to resurrect, at least in a bilateral form, parts of that Bush-era program. The matter has yet to hit the political radar screen in the United States. When journalists were briefed by a State Department official about the visit to Canada by Hillary Clinton in mid-December, where she met with her Canadian and Mexican counterparts, no one mentioned the talks. They were more interested in whether India's ambassador to the UN had his turban searched by airport security in Houston. Among the many things Mr Harper will have to contend with if he wants his new deal is American indifference.

Photo credit: bbmcshane via Flickr