For decades, people living in communities along the Canada–US border enjoyed close social and economic relationships with their neighbours across the line. The introduction of new security measures during the First World War threatened this way of life by restricting the movement of people and goods across the border. Many Canadians resented the new regulations introduced by their provincial and federal governments, deriding them as “outside influences” that created friction where none had existed before. Engaging the Line examines responses to wartime regulations in six communities and offers a glimpse at the origins of our modern, highly secured border.