A fear that America's obsession with security might gum up critical border travel has loomed over some of Canada's domestic-policy debates. But on two matters currently in the news – the legalization of marijuana and visa-free travel for Mexicans – the United States is proving not to be the border bogeyman that Canadian politicians and bureaucrats sometimes make it out to be.

Last week, as the presidents of Mexico and the U.S. visited Ottawa, Prime Minister Justin Trudeau announced he will lift the requirement that Mexicans have a visa to visit Canada. This move caused controversy, as bureaucrats raised concerns of a "significant risk" the U.S. will see Canada as weak on security and decide to "thicken" border regulations.

But U.S. Ambassador to Canada Bruce Heyman says his country has no such concerns – and he says he even went out of his way to tell the previous, Conservative government that.

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Politicians and pundits have, over the years, also expressed fear that legalizing marijuana will spark a U.S. border slowdown that would hurt trade and travel. But as Mr. Trudeau's government announced a task force on legalizing pot Thursday, Mr. Heyman insisted the border issues can be worked out, and noted some U.S. states have voted to legalize marijuana, too.

"Each country is going to have to decide their own drug policy," Mr. Heyman said.

At the North American leaders' summit Wednesday, Mr. Trudeau, U.S. President Barack Obama and Mexican President Enrique Pena Nieto agreed to redouble efforts to combat opioid abuse, the ambassador noted, but he said he's never had any high-level discussions about Canada's legalization plans or a possible impact on the border.

"That has not been something that has come up," he said. "They're not red flags yet. I don't anticipate they are. But I'm sure it will be a point of discussion as we'll have to deal with that at the border – but then again … some states are dealing with these issues, early on, in the United States as well. … Look, if Canada sets a certain set of laws, we'll work with Canada to try to make sure [it works]."

No U.S. government official will endorse the Trudeau government's plans to legalize marijuana. Pot is illegal in the U.S., and the Obama administration's policy is that it should stay that way, even though four states have legalized it, and a dozen more have decriminalized its use.

And of course, the nature of Canadian legalization might affect the U.S. reaction. Mr. Heyman didn't discuss details, because Mr. Trudeau's government hasn't revealed them yet. But it's not hard to imagine that the U.S. would have greater concern about drug smuggling if Canada completely legalized marijuana production, allowing anyone to set up a grow op. The Trudeau government has signalled it wants tight controls on growing pot, however.

There's certainly no doubt the U.S. remains deeply conscious of border security, especially about preventing terrorists from entering the country, as well as serious criminals. Those post-9/11 border-security measures taught Canadians to worry about the effects of a thickening border.

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But those fears sometimes serve as an easy strawman in domestic policy debates.

Those concerns were misplaced in the case of Mexican visas. Mr. Trudeau promised to lift the visa requirement immediately during last year's election campaigns, but he has now delayed it till December. Bureaucrats, according to a memo leaked to the CBC, raised concerns, including a "significant risk" that if the visa requirement were scrapped, the U.S. would see Canada as weak on security, and crack down at the border.

Mr. Heyman said he heard of the same concerns when the Conservatives were in power, and tried to dispel them. "It is not a U.S. issue," he said.

"I meet so frequently with [Customs and Border Protection] and Homeland Security officials on so many things. I have never had a discussion with CBP or Homeland Security with regard to any concern or any issue or anything that's come up with regard to Mexican visas," he said. "Never."