US Army enlistment records do not merely document military service; they can shed light on otherwise obscure aspects of American immigration history. This is especially true in regard to the Canadian-born men who joined the forces of the republic during the Mexican War (1846–48). Newly retrieved data indicates that approximately 1,500 individuals from British North America enlisted—a fairly widespread survival strategy at a time when people from the Canadas and the Atlantic colonies were increasingly seeking opportunities abroad. Enlistment records tell us something of the economic backgrounds of the prospective soldiers and the ways in which the creation of a national market in the United States drew British subjects south of the border. Thus, they provide a valuable glimpse into an oft-overlooked aspect of Canadian and North American history prior to 1849.