It was concern for defence that inspired construction of the Rideau Canal. The United States declared war on Britain in 1812, and the conflict dragged to a stalemate in 1814. Though Canada was preserved for Britain, military authorities in London were uneasily aware that the border where it ran along the St. Lawrence represented a weak point in their defences.

Had the enemy set up batteries facing Canada across the narrow upper reaches of the St. Lawrence, Britain would have lost access to Upper Canada, and the territory would have fallen to the enemy. After the war, the military pushed for creation of an alternate route to carry men and munitions westward at a safe distance from the American border. The project, which a cash-strapped British government finally launched in 1826, was called the Rideau Canal.