The United States and Canada share the longest demilitarized border in the world. On one awful day three weeks ago, the border long hailed by both sides as open became criticized as porous, talk suddenly turned from increasing the free passage of people and products to tightening security.

The link between the Sept. 11 terrorist attacks and the U.S.-Canadian border is still speculative – the most concrete evidence

is that a man with connections to the hijackers and a history of Canadian immigration violations is in custody after being detained by the FBI in Chicago. The potential for this border to attract terrorists was exposed as long ago as late 1999, when an Algerian who had been living in Canada was caught trying to cross into the United States at the Washington state border with explosives in his trunk.

The attacks put a border often considered little more than a slightly more involved tollbooth under a spotlight. Attorney General John Ashcroft noted, for example, that while 9,000 Border Patrol agents work the Mexican border, only 500 are stationed on the Canada line. That force will be increased. Inspections of vehicles and luggage will intensify. The ability of U.S. border police to quickly access law-enforcement data banks to check out travelers will be enhanced.

After such immediate unilateral steps must come a concerted, binational effort. That can come only after each of the two countries gains a thorough understanding of the other’s policies, procedures and underlying philosophies.

Maine Sen. Olympia J. Snowe, long a critic of the understaffed northern border, is asking the U.S. Immigration and Naturalization Service for a detailed evaluation of how well U.S. and Canadian border practices mesh and how the immigration, asylum and refugee laws compare. From this needed assessment will come the ability to determine how cooperation can be increased in the exchange of law-enforcement information, resources and technology.

The importance of this border and its vulnerability is also at the top of Ottawa’s agenda. Legislation already is before Parliament that will impose tougher standards and screening for those claiming refugee status. The government may take over airport security, or at least take on a bigger role. Although Canada has a speedier process than the United States for expelling immigrants convicted of crimes, even faster action is under consideration. The call for a comprehensive anti-terrorism plan is every bit as loud there as here.

Given the length of the border and the budget realities on both sides, it is necessary and inevitable that greater security will require a considerable degree of joint action. Despite chronic concerns about being subsumed by the giant to their south, a nationwide poll finds 85 percent of Canadians in favor of establishing a North American security perimeter, 70 percent favor joint border posts staffed by U.S. and Canadian officers, 81 percent think Canada and the United States should adopt the same entry controls.

The obvious logic to the idea of considering North America as one security region governed by consistency, cooperation and clear expectations does not mean common control over borders and immigration. The discrepancy between the policies the two countries have toward Cuba provides an obvious example of the type of foreign policy clash that would occur.

It comes as no surprise to any American with even the most passing knowledge of Canada that sovereignty is cherished as much there as here; the success of the efforts under way to improve security must strike the proper balance between pursuing mutual interests and preserving national identities. Fortunately, the tradition of cooperation between the two countries is nearly as long as the border they share.