At 5,525 milesdo888, it is the longest border between any two countries. And that border — separating the United States and Canada — seems set to become a flashpoint between the close allies as President-elect Donald J. Trump prepares to take power.

The Canadian authorities fear that Mr. Trump’s promised mass deportations will push migrants north, while allies of the incoming president headed for key roles in his administration have raised alarms over a recent spike in undocumented migrants crossing from Canada to the United States.

Canadian officials are drawing up plans to add patrols, buy new vehicles and set up emergency reception facilities at the border between New York State and the province of Quebec to prepare for what they expect to be a surge in migrants because of Mr. Trump’s hard line on deportations.

The northern border is also a focus of people named to top positions in the Trump administration, including his new border czar, Tom Homan. He has described the frontier as a major security vulnerability because of what he described as insufficient checks on people entering the United States.

Concerns over the northern border highlight a key shift in Canada’s view of migration between the first and second Trump administrations. During the first, Prime Minister Justin Trudeau of Canada made clear that asylum seekers were welcome in Canada just as Mr. Trump was tightening asylum rules and introducing policies like family separation at the U.S. southern border.

“To those fleeing persecution, terror & war, Canadians will welcome you, regardless of your faith. Diversity is our strength #WelcomeToCanada,” Mr. Trudeau posted on social media at the time.

The map locates Plattsburgh, New York, and Pittsburg, New Hampshire, as well as the Roxham Rd. border crossing between the U.S. and Canada.

250 miles

Detail

area

Canada

Me.

Toronto

N.H.

Brampton

N.Y.

VT.

United States

New York City

40 miles

Montreal

Quebec

Roxham Rd.

border

crossing

3

Pittsburg

Plattsburgh

New

Hampshire

New

York

Vermont

By The New York Times

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