At 5,525 miles, 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.
