Not Legal Advice: The term “posse comitatus,” literally translated as the power of the county first appeared in English law in 1411 with the passage of a riot act calling for the sheriffs and justice of the peace together with the “poair de counte” to arrest rioters.
In the United States, the posse comitatus was an important institution on the western frontier, where it became known as the posse. At various times vigilante committees, often acting without legal standing, organized posses to capture wrongdoers.
Such posses sharply warned first-time cattle rustlers, for instance, and usually hanged or shot second-time offenders. In 1876 a four-hundred-man posse killed one member of the infamous Jesse James gang and captured two others. - Legal Dictionary - Posse Comitatus.
In 1878 the use of a posse comitatus was limited by the passage of the Posse Comitatus Act of 1878.
Posse Comitatus Act restricts the participation of the military in domestic law enforcement activities under under many circumstances. The text of the relevant legislation is as follows:
Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.
Refer to Rand.org - OVERVIEW OF THE POSSE COMITATUS ACT to learn more.