The two continental borders of the United States are a study in contrasts.

To the south, across barren deserts, dusty scrubland and the Rio Grande is Mexico. This is where Donald Trump wants to build a wall, ostensibly to keep the human flood of cheap labor from flowing north—even though, as Pew found in 2015, more Mexicans are leaving the U.S. and headed south than coming north for “opportunity,” meaning the wall’s true purpose may be to keep those low-paid workers in.

To the north, past pine and oak forests and snowy windswept plains, is Canada, America’s hat, America’s kid sibling, the charming, harmless and somewhat-sappy butt of numerous jokes taking advantage of its good nature. This is where Trump might better consider building a wall—in order to keep Americans in.

It’s not that the majority of Americans who voted for someone else for president are at last fleeing the country for political reasons, or that celebrities are making good on that vow to escape to Canada now that Trump’s once-unthinkable election has come to pass.

According to an Ottawa, Canada-based immigration lawyer, more and more Americans with cannabis industry experience are seeking work permits in Canada, in order to participate in that country’s state-sanctioned experiment with marijuana legalization. As the Huffington Post Canada reported, that’s only the beginning of how U.S.-Canadian relations may change thanks to Canada’s full-on embrace of marijuana legalization.

Along with Israel, Canada is already one of the world’s most marijuana-friendly countries. Canada is the only national government to issue permits and licensing to medical marijuana producers, and this spring, Prime Minister Justin Trudeau’s Liberal government is expected to unveil long-awaited legislation that would legalize recreational cannabis in Canada.

Suffice to say that the incoming Trump administration feels differently. Trump has named a staunch drug war advocate, Alabama Sen. Jeff Sessions, to lead his Justice Department, and Trump spent a good portion of his ominous speech at the Republican National Convention this summer calling for “law and order.” While Trump’s plan for his country’s burgeoning, multibillion-dollar cannabis industry is completely unclear, nothing he has said or done to date is encouraging.

It may be hard to imagine cannabis causing friction along the border in an age when legalization is sweeping the country, but marijuana has frayed U.S.-Canada relations before.

In the early 2000s, then-Liberal Prime Minister Jean Chrétien angered American officials when he and other drug reform advocates first called for Canada to liberalize its drug laws. George W. Bush’s drug czar lamented the “poison” that would flow south should Canada decriminalize (though Canada’s refusal to support America’s bellicose stance on Iraq may have been behind that beef).

And eventually, the pressure worked: Canada backed down, and progress was slowed down by more than a decade.

Canada will have to test the U.S.’s resolve on this again. As the Post pointed out, Canada is one of the signatories to a United Nations treaty on narcotics that declares cannabis an unmitigated evil. Backing out of the treaty will require some skillful diplomacy, though it bears mentioning that Israel has managed to lay plans for a marijuana export economy without drawing American ire. (Then again, Israel enjoys a very special relationship with the U.S.)

With so much domestic marijuana available in the U.S.—California supplies as much as 75 percent of the country’s black market cannabis, according to a 2010 estimate from federal drug officials—there may not be much apparent need for Canadian cannabis. But some will surely flow south, especially to areas where recreational marijuana is not available in the U.S., such as Minnesota and Michigan, the Huffington Post suggested.

In addition to drawing migrant marijuana workers, Canada will also surely attract tourists. With the drinking age at 18 in Quebec and 19 in the rest of the country, Canada is already a magnet for American college students looking to escape the perdition of a 21-and-up drinking age. It won’t draw many visitors from California or Washington, where recreational marijuana is legal, but other parts of the border could see an influx of weed-seeking day-trippers.

And lest you think any kind of international incident around marijuana is unlikely, remember: there are already border tensions around cannabis. As was widely reported earlier this year, U.S. Customs and Border Patrol officials have denied dozens of Canadians entry into the U.S. after they admitted using marijuana. If Trump-loving, law-and-order types want to, they could make things difficult for Americans returning from North America’s new cannabis paradise, too.

Related: Whoa, Canada! Retail Marijuana Market Could Top $4.5 Billion

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