How does manpower affect counterinsurgency? Important debates about counterinsurgency theory, military doctrine, force planning, and ongoing military operations revolve around assumptions about the role manpower plays in determining counterinsurgency outcomes. But these assumptions have not, by and large, been subjected to large-n analysis. This paper helps serve that role by examining new data on counterinsurgents‘ deployments across 171 campaigns since World War I. These data provide insight into a range of important issues, such as how force size should be measured, whether it is related to counterinsurgent success, whether troop nationality matters, and whether the role of manpower varies across contexts. Of these findings, the most notable is that conventional rules of thumb for force sizing, including the recommendation put forth in official U.S. military doctrine, receive no empirical support. These findings therefore challenge the prevailing wisdom, while laying the groundwork for a range of future scholarship.