Canadian Prime Minister Mark Carney admitted last week that he was preparing for a once unimaginable scenario - a U.S. invasion.
He pointed out that the risk was low and the possibility was minimal, but told the New York Times that the relationship between the United States and Canada is being redefined under the administration of Donald Trump, and it is his responsibility to consider all possibilities.
"This is just risk management," Carney said.
Trump has imposed tariffs around the world and repeatedly claimed that he would make Canada the 51st state of the United States, prompting Canada to conduct war games against the United States.
In 2025, the US president said it was "extremely unlikely" that the US would use force against Canada. Instead, he said, he would continue to apply economic pressure to achieve political goals.
"President Trump has done more than anyone to protect the great people of Canada," the White House said in a statement to the BBC this week.
Military experts interviewed by the BBC also believe that the possibility of invasion is extremely low. They pointed out that the two countries have many ways to cooperate in defense.
But the question is clearly lingering on Canadians' minds.
In January, after the United States captured Nicolás Maduro in Venezuela, a Leger poll showed that more than 30 per cent of Canadians believed a U.S. invasion was possible.
Why an invasion is unlikely
Lieutenant General Christopher Coates said a war between Canada and the United States would be extremely complex and almost unimaginable. He served in the Royal Canadian Air Force for 34 years, holding various senior positions.
"I have been involved in military operations my whole career and we never considered a military attack from American soil," he told the BBC.
Coats, who has participated in Nato operations, pointed out that Canada and the United States cooperate on many aspects of military defense.
Both countries are members of this North Atlantic defense alliance. The alliance stipulates that if a member country is attacked, other members must defend it.
The two countries also perform joint Arctic defense missions within the framework of the North American Aerospace Defense Command (NORAD) and frequently share intelligence.
Therefore, Coates said that he believes that if the United States takes action against Canada, it will not start with an armed attack, but will start with more covert means such as cyber warfare.
not always best friends
After World War II, Canada and the United States became natural allies in the second half of the 20th century and were highly integrated economically, militarily, and culturally.
But that wasn't always the case. In fact, resisting American expansionism in the 18th and 19th centuries was one of the core tenets of Canadian identity, said Tim Foran, a researcher at the Canadian War Museum.
"Fear of the United States is the foundation of this political project in Canada," he said.
Canada was a group of British North American colonies that chose to be loyal to the British crown when the colonies in the United States rose up against the British today.
Before the founding of Canada, the United States attempted to invade the British North American colonies, capturing Montreal in 1775 and nearly capturing Quebec City.
During the War of 1812, the Americans repeatedly invaded what was then known as Upper Canada, Lower Canada and the coastal colonies, eventually reaching a stalemate. During that war, British troops retaliated by burning down the White House, an event that became famous throughout the world.
Foran said the colonies that would become Canada relied on the British Empire and its Royal Navy for defense, as well as Aboriginal people who served as generals in major battles.
After World War I, faced with the growing power and influence of the United States, Canada once again considered the possibility of war with the United States.
Canada had drafted a plan based on secret trips made by Canadian military generals to border cities, where they pretended to be tourists and took photos of bridges and highways to gather intelligence.
They also talked to local residents to better understand their enemies.
Canadian War Museum researcher Andrew Burtch is an expert on 20th century Canadian history. War was unlikely at the time, he said, but Canadians still wanted to be prepared.
"That's part of what the military does," he said. "They have to imagine all kinds of different scenarios."
What an American invasion might look like
Matthew Schmidt, an associate professor of national security at the University of New Haven in Connecticut, said that even if a U.S. invasion is highly unlikely, the very fact that Canadians are preparing for it shows how drastically the global order has changed.
He added that Canadian leaders do face some tough questions: How closely they want to continue to work with the U.S. military — a partnership that would give the U.S. valuable information that could be used if the U.S. wanted to harm Canada in any way.
"This is really damaging to the relationship with the United States, and a certain level of fear is legitimate," he said.
He said Americans may not take Trump's words about Canada becoming the 51st state seriously, but Canadians clearly do - and they are prepared to defend themselves if necessary.
So, what would happen if this really happened?
The first phase of the war can be quite brief.
The Globe and Mail reported earlier this year that, according to models used by the Canadian military, a U.S. offensive would overwhelm Canada's strategic positions within a week.
The Canadian Army will quickly be outnumbered and forced to resort to guerrilla-style warfare in an attempt to suppress the enemy.
The newspaper reported in January this year, citing senior government officials, that Canada would request support and defense from allies such as France and the United Kingdom.
This theoretical model is believed to be the first time in a century that Canada has developed plans for a U.S. attack.
Coates said that in terms of military operational planning, this model is on the lighter end of the spectrum.
He said Canada, unlike its NATO allies, has lagged behind in planning for any ground invasion and is now playing catch-up in an era of unpredictable geopolitics.
He said this was evident in Canada's chief of defense staff's recent call for a significant expansion of the military - including up to 300,000 additional lightly trained citizen soldiers.
From a U.S. perspective, invading Canada might not be easy either.
Schmidt said Canadian and American soldiers often train together and have a mutual respect for each other.
While he acknowledged that the United States has greater military capabilities, he said, all things being equal, Canadian soldiers are capable of fighting it.
