One hallmark of US-Canadian friendship has been "the longest undefended border in the world." No more. The antiterrorism pact recently announced in Ottawa by US Attorney General John Ashcroft made that clear.

Shocking some of his Canadian hosts, Mr. Ashcroft revealed that among the steps being considered is the use of National Guard troops and helicopters to beef up surveillance of the 4,000-mile line. He also said such a step would be temporary - though with this, as with other security measures in this unprecedented kind of war, it's hard to know what "temporary" means.

What must remain permanent, certainly, is the close relationship between two North American neighbors. That relationship, in fact, should be cemented even more tightly by the current threat. Other, more important parts of Ashcroft's agreement with Canada involved expanded sharing of intelligence and closer teamwork between law-enforcement agencies.

On its own, Canada has wisely strengthened some of its laws to allow more effective investigation of terrorist cells. The existence of such cells in Canada has been well documented.

Even as it joins forces with the US, Canada hopes a security buildup doesn't impede cross-border commerce - which happened just after Sept. 11. To work to both countries' benefit, the antiterrorism thrust has to keep that issue clearly in view.

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