Today the borderland between Canada and the United States is a wide, empty sweep of wheat fields and pasture, measured by a grid of gravel roads that sees little traffic and few people who do not make their lives there. It has been much this way for more than a century now, but there was a moment when the great silence shrouding this place was broken, and that moment changed it forever. Arc of the Medicine Line is a compelling narrative of that moment—the completion of the official border between the United States and Canada in 1874. In late July of 1874, the Sweetgrass Hills sheltered the greatest accumulation of scientists, teamsters, scouts, cooks, and soldiers to be seen in this part of the world before the coming of the railways. The men of the boundary commissions—American, British, and Canadian—established an astronomical station and the last of their supply depots as they prepared to draw the Medicine Line across the final hundred of the nearly nine hundred miles between Manitoba’s Lake of the Woods and the Continental Divide. In the brief weeks the surveyors and soldiers spent in Milk River country, they witnessed, and played a singular part in, the beginning of the end for the open West. That hot, dry summer of 1874 marked the outside world’s final assault on this last frontier.