The United States and Canada today share the longest undefended border in the world, encompassing 5,525 after the US purchased Alaska, and though they have long been allies, the border has not always been peaceful. During colonial times, generations of the British war with France meant generations of threats and actual attacks by Canadian militia and allied Indians from New France. The British ended that threat from Canada by defeating France in 1763 and forever ending French rule in North America. When the 13 colonies rebelled against British rule, the Canadian colonies remained loyal, despite invitations from the American rebels and an invasion by American forces. When the Revolution ended, tens of thousands of Loyalists fled to Canada as refugees, strengthening loyalty to Britain. In the War of 1812, American forces again invaded Canada and achieved several victories, but retreated in the face of British forces, a harsh climate, and Canadian loyalty to Britain. 

Those were the last big invasions, but it took generations for the demilitarized border to become completely peaceful like it is today. Disenchanted Canadians, Canadian rebels, and American sympathizers led incursions into Canada in the 1830s. The violence of the 1830s within Canada itself encouraged American groups to dabble, and Canadian radical reformers took refuge on the American side of the border as they plotted. 

Violent Irish revolutionaries known as the Fenians had found refuge in the United States and connected Canada with the cause of Irish independence, resulting in yet more incursions in 1866 and 1871. The Fenians even financed a submarine in 1881, hoping it could be used against British warships based in Halifax, Nova Scotia. The hope was that war with the United States might somehow achieve Irish independence. There were also border disputes that had considerable potential to lead to war, and the US had detailed plans to invade Canada as recently as the 1930s. 

The political geography is important. The bulk of the Canadian population has always been in a narrow band extending about a hundred miles north of the border. Canadian trade relies heavily on the Great Lakes and St. Lawrence River estuary, so the connections between the various Great Lakes become natural choke points. So do man-made connections such as the Soo Locks, and the Welland Canal that allows shipping to bypass the natural block of Niagara Falls. The rail and communications infrastructure that connects eastern Canada with the Prairie Provinces and British Colombia are all close to and parallel with the border, and could easily be cut. 

In addition, the economic and industrial heart of Canada is the Ontario peninsula, easily invaded from the west from Michigan, and equally easily invaded from the east, across the Niagara frontier, from New York. The sole danger of invasion from a Canadian enemy has always come from south of the border - from the United States.