The War of 1812 was fought between the United States and the British, with the British North American colonies being heavily involved. Greatly outgunned by the British Royal Navy, the American war plans focused on an invasion of Canada (especially what is today eastern and western Ontario). The American frontier states voted for war to suppress the First Nations raids that frustrated the settlement of the frontier. The war on the border with the United States was characterized by a series of multiple failed invasions and fiascos on both sides. American forces took control of Lake Erie in 1813, driving the British out of western Ontario, killing the Shawnee leader Tecumseh, and breaking the military power of his confederacy. The war was overseen by British army officers like Isaac Brock and Charles de Salaberry with the assistance of First Nations and loyalist informants, most notably Laura Secord.

The War ended with no boundary changes thanks to the Treaty of Ghent of 1814, and the Rush–Bagot Treaty of 1817. A demographic result was the shifting of the destination of American migration from Upper Canada to Ohio, Indiana and Michigan, without fear of Indigenous attacks. After the war, supporters of Britain tried to repress the republicanism that was common among American immigrants to Canada. The troubling memory of the war and the American invasions etched itself into the consciousness of Canadians as a distrust of the intentions of the United States towards the British presence in North America.