Is Donald Trump right when he says the border is just an 'artificially drawn line'?

U.S. President Donald Trump repeated one of his favourite talking points in his meeting with Prime Minister Mark Carney Tuesday, saying the Canada-U.S. border is an "artificially drawn line."

"Somebody drew that line many years ago with, like, a ruler — just a straight line right across the top of the country," he said when the two leaders met in front of reporters at the White House.

When a reporter later asked Carney what he was thinking when Trump made the comment, the prime minister quipped, "I'm glad that you couldn't tell what was going through my mind."

Trump has frequently called the border line "imaginary" when musing about annexing Canada.

Canadian history experts say establishing the Canada-U.S. border was, in fact, a long and complex process that involved numerous treaties and took more than a century.

However, they say, Trump does have a point.

"He's just trying to use that to cause chaos and to provoke annoyance to people, and to stir the pot," said Stephen Bown, author of Dominion: The Railway and the Rise of Canada. "But from a historian's point of view, he's not inaccurate, either."

WATCH | Mark Carney asked about Trump's border comments:

Speaking Tuesday from the roof of the Canadian Embassy in Washington, Prime Minister Mark Carney said 'I'm glad you couldn't tell' what was going through his mind when he heard President Donald Trump calling the Canada-U.S. border 'artificial' during their sit-down in the Oval Office.

Bown says a lot of international boundary agreements from the 19th century are "somewhat nonsensical" because they were signed by people who didn't know exactly what they were agreeing to.

The drawing of borders between the United States and British North America effectively began in the east with the Treaty of Paris in 1783, following the American Revolution.

Many treaties followed in the ensuing decades, but it was the Treaty of 1818 that began the long push west, drawing a line across the 49th parallel as British North America and the U.S. expanded — in part because the straight line would be easier to survey than the pre-existing boundaries that were based on watersheds and other natural features.

"When all the lines were just being randomly drawn upon maps by people in conference rooms, often in Europe or in Washington, between various diplomats, none of these people had ever been to any of the land that they were marking up," Bown said.

"The maps that they were working from were completely inaccurate, because there weren't significant numbers of European-descended settlers living in a lot of that land, especially in the West, during those time periods."

In many cases, the lines bisected through traditional lands of Indigenous Peoples. The Blackfoot Confederacy, for example, stretched through what is now the Canadian Prairies and Montana.

The Oregon Treaty of 1846 settled a dispute between the British and the Americans, again using the 49th parallel to cut through the Rocky Mountains to the pacific coast, completing the westward push.

Bown says many of the claims to land were made by people who had "no real authority" to make them in the first place.

In 1869, for example, Canada's first prime minister John A. Macdonald facilitated the transfer of Rupert's Land, spanning much of what is now eastern and central Canada, from the Hudson's Bay Company for £300,000.

WATCH | Carney tells Trump Canada is not for sale:

During a meeting in the Oval Office, Prime Minister Mark Carney told U.S. President Donald Trump that Canada will never be for sale, going on to say that the opportunity between the two nations ‘is in the partnership and what we can build together,’ including around security. Trump, who has repeatedly raised the notion of Canada as the 51st state, added ‘never say never.’

"In what sense the Hudson's Bay Company have any title to the land? They didn't," Bown said. "They just recognized, 'Britain wants to pretend that we do, and they're going to pay us some money if we say that we do, so OK.' And that land became part of Canada."

He says Manifest Destiny — the Americans' belief that they were destined by God to expand westward — threatened to take British Columbia until Macdonald's promise of a railway lured the colony to join the Canadian Confederation in 1871.

Bown says it's easy today to translate old border agreements onto modern maps, but much of the actual land along the borders wasn't surveyed until a generation after the agreements were signed.

Among the last major redrawings of the Canada-U.S. border happened in 1908, when the southeast border of Alaska, among other borders, was negotiated between the U.S., the U.K. and Canada.

Craig Baird, host of the podcast Canadian History Ehx, says the U.K. was trying to develop a better relationship with the U.S. at that time, giving the Americans a favourable outcome.

"That's why there's a large chunk of that Alaska panhandle, including Juneau, that actually is part of the United States and not part of Canada," he said. "And it's also a reason why the Yukon doesn't have access to the Pacific Ocean. That was a big sticking point, that we really wanted Yukon to have some sort of access to the Pacific Ocean."

Baird says disputes over the Canada-U.S. border have generally been settled peacefully through treaties. But after centuries of tweaks and skirmishes, he says that invisible line is "pretty much carved into stone now."

The Canada-U.S. border is the world's longest undefended border, stretching almost 9,000 kilometres across land and water.

Redrawing it in the 21st century, Baird says, would be nearly impossible.

"It is something that has been there for a long time, and it's not going to change," he said.

"You can't just erase a line and redraw it and say, 'This is how it's going to be.'"