Courts and legislatures in colonial America and the early American republic developed and refined a power to compel civilians to assist peace and law enforcement officers in arresting wrongdoers, keeping the peace, and other matters of law enforcement. This power to command civilian cooperation was known as the posse comitatus or “power of the county.” Rooted in early modern English countryside law enforcement, the posse comitatus became an important police institution in 18th- and 19th-century America. The posse comitatus was typically composed of able-bodied white male civilians who were temporarily deputized to aid a sheriff or constable. But if this “power of the county” was insufficient, law enforcement officers were often authorized to call on the military to serve as the posse comitatus. The posse comitatus proved particularly important in buttressing slavery in the American South. Slaveholders pushed for and especially benefited from laws that required citizens to assist in the recapture of local runaway slaves and fugitive slaves who crossed into states without slavery. Though slave patrols were rooted in the posse comitatus, the posse comitatus originated as a compulsory and noncompensated institution. Slaveholders in the American South later added financial incentives for those who acted in the place of a posse to recapture slaves on the run from their owners. The widespread use of the posse comitatus in southern slave law became part of the national discussion about slavery during the early American republic as national lawmakers contemplated how to deal with the problem of fugitive slaves who fled to free states. This dialogue culminated with the Fugitive Slave Law of 1850, in which the US Congress authorized officials to “summon and call to their aid the bystanders, or posse comitatus” and declared that “all good citizens are hereby commanded to aid and assist in the prompt and efficient execution of this law, whenever their services may be required.” During Reconstruction, the Radical Republican Congress used the posse comitatus to enforce laws that targeted conquered Confederates. After the end of Reconstruction in 1877, Southern states pushed Congress to create what would come to be known as the “Posse Comitatus Act,” which prohibited the use of federal military forces for law enforcement. The history of the posse comitatus in early America is thus best understood as a story about and an example of the centralization of government authority and its ramifications.