This isn't the first time American leaders have suggested annexing Canada

U.S. president-elect Donald Trump says he's prepared to use economic force to turn Canada into America's 51st state, and it's making Canadians — two-thirds of whom believe he's sincere — anxious. 

But the last time Canada faced the threat of American annexation, it united us more than ever before, leading to the foundation of our country as we know it today.

In the 1860s, several prominent U.S. politicians advocated for annexing the colonies of British North America. 

"I look on Rupert's Land [modern-day Manitoba and parts of Alberta, Saskatchewan, Nunavut, Ontario, and Quebec] and Canada, and see how an ingenious people and a capable, enlightened government are occupied with bridging rivers and making railroads and telegraphs," Secretary of State William Henry Seward told a crowd in St. Paul, Minn. while campaigning on behalf of presidential candidate Abraham Lincoln.

"I am able to say, it is very well; you are building excellent states to be hereafter admitted into the American Union."

Seward believed in Manifest Destiny, the doctrine that the United States would inevitably expand across the entire North American continent. While he seems to have preferred to acquire territory through negotiation rather than aggression, Canadians weren't wholly assured of America's peaceful intentions. 

The War of 1812, when the U.S. had launched a three-pronged invasion into Canada, was still in living memory, and as recently as the 1840s the U.S. had invaded Mexico and annexed California, Nevada, Utah and New Mexico, as well as most of Arizona and Colorado.

During the American Civil War, the Union army grew to a strength of one million men, equivalent to a third of the population of Canada, Nova Scotia and New Brunswick. If the U.S. were to invade, as Amelia Harris of London, Ont., confided in her diary in 1864, "the country will be overrun."

When the Civil War drew to a close, some American politicians did indeed begin calling for a renewed focus on territorial expansion.

On July 2, 1866, Congressman Nathaniel Prentice Banks introduced a bill in the U.S. House of Representatives authorizing the president to annex New Brunswick, Nova Scotia, Prince Edward Island, Newfoundland, Canada [Ontario and Quebec], British Columbia and Vancouver Island.

Meanwhile, a group of Irish-Americans known as the Fenians really did invade Canada and New Brunswick with the intention of seizing territory they could ransom to Great Britain in exchange for Irish independence. 

Though these small raids failed to hold any ground, they gave life to the fear of American incursions onto Canadian soil.

The external threat posed by the U.S. spurred the francophone and anglophone factions in the government of Canada to look beyond their domestic squabbles and think instead of their sovereignty. Confederation between the provinces of British North America moved to the top of their agenda. 

In an 1864 speech in Halifax, Member of Parliament Thomas D'Arcy McGee framed Confederation as the only way to secure the colonies.