The Posse Comitatus Act of 1878 generally prohibits military forces from performing ordinary civilian law enforcement functions such as arrest, surveillance, interdiction, search and seizure.
But a newly updated Department of Defense doctrinal publication notes that, despite this prohibition, “There are several forms of direct assistance to civilian law enforcement by military personnel that are permitted under the Military Purpose Doctrine. The Military Purpose Doctrine provides that law enforcement actions that are performed primarily for a military purpose, even when incidentally assisting civil authorities, will not violate the PCA [Posse Comitatus Act].”
These may include investigations related to the Uniform Code of Military Justice, enforcement actions on a military installation, and measures to protect classified military information or equipment, among others identified by the DoD document.
The publication broadly addresses crisis response, support to law enforcement, and other forms of assistance. See “Defense Support of Civil Authorities,” Joint Publication 3-28, July 31, 2013.
The publication introduces a new addition to the DoD lexicon: “complex catastrophe.”
A complex catastrophe (which may “magnify requirements for defense support of civil authorities”) is defined as: “Any natural or man-made incident, including cyberspace attack, power grid failure, and terrorism, which results in cascading failures of multiple, interdependent, critical, life-sustaining infrastructure sectors and causes extraordinary levels of mass casualties, damage or disruption severely affecting the population, environment, economy, public health, national morale, response efforts, and/or government functions.”