Often described as the longest undefended border in the world, the Canada-US border was born in blood, conflict, and uncertainty. At the end of the American Revolution, Britain and the United States imagined a future for each of their nations that stretched across a continent. They signed treaties with one another dividing lands neither country could map, much less control. A century and a half later, Canada and the United States had largely fulfilled those earlier ambitions. Both countries had built nations that stretched from the Atlantic to the Pacific and had made an expansive international border that restricted movement. The vision that seemed so clear in the minds of diplomats and politicians never behaved as such on the ground. Both countries built their border across Indigenous lands using hunger, violence, and coercion to displace existing communities and to disrupt their ideas of territory and belonging. The border's length undermined each nation's attempts at control. Unable to prevent movement at the border's physical location for over a century, Canada and the United States instead found ways to project fear across international lines They aimed to stop journeys before they even began.