President-elect Trump has doubled down on his suggestion of a merged United States and Canada in the wake of Canadian Prime Minister Justin Trudeau’s decision to resign.

Some have shrugged off Trump’s suggestion that Canada should “become the 51st state” as an old joke or mere posturing, while others dismiss it as a bad political idea, given Canada’s blue leanings.

But Trump on Tuesday threatened “economic force” to annex the U.S.’s neighbor to the north. Together with talks of buying Greenland and controlling the Panama Canal, the comments suggest he’s aiming to expand U.S. power after he takes office this month.

The concept, though, continues to be largely unpopular among Canadians. And with Trudeau exiting the political stage, it’s unclear what impact Trump’s remarks will have on the U.S.-Canada relationship under new leaders.

Here’s what to know about Trump's calls to merge the U.S. and Canada:

Annexation efforts date back to early U.S. history

The idea of a union between the U.S. and Canada dates back centuries, when early American leaders invaded Quebec during the American Revolution. A few decades later, the U.S. unsuccessfully invaded Canada during the War of 1812 amid frustration over the Royal Navy’s moves to bring American sailors into their own ships.

“Canada and the US fought a war over this in 1812,” said Matthew Lebo, a political science professor at the University of Western Ontario and a visiting professor at McGill University, as he shrugged off Trump’s idea of a renewed annexation push.

Annexation and invasion fears continued through the American Civil War and amid the U.S. push for expansion and Manifest Destiny, or the idea that the US was destined to control the continent. But those fears ended as the two nations shifted to become “fierce allies” in the 20th Century, said Duane Bratt of Mount Royal University, a political science professor with a focus on Canadian foreign policy.

“NORAD was formed. NATO was formed. Canadians fought with Americans in World War I and in World War II, and in Korea and in the Gulf War,” Bratt said. “American soldiers died for Canada. Canadian soldiers died for the........

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