posse comitatus: [ Latin, Power of the county. ] Referred at Common Law to all males over the age of fifteen on whom a sheriff could call for assistance in preventing any type of civil disorder. The notion of a posse comitatus has its roots in ancient English Law, growing out of a citizen's traditional duty to raise a "hue and cry" whenever a. On April 3, 2018, President Donald Trump proposed that U.S. military troops be deployed along the United States border with Mexico to help control illegal immigration and maintain civil order during construction of the secure, border-length fence recently funded by Congress. The proposal brought questions of its legality under the 1878 Posse Comitatus Act.