Can the Marines gather intelligence related to threats involving domestic and foreign communication?

Generally, the U.S. Marines, along with other branches of the federal military, cannot be used for domestic law enforcement purposes due to the Posse Comitatus Act. This act, passed in 1878, limits the use of the military in civilian law enforcement unless Congress approves it or under circumstances "expressly authorized by the Constitution".

However, exceptions exist, primarily through the Insurrection Act. This act permits the President to deploy federal troops, which now includes the Marines, in specific scenarios. These situations include suppressing an insurrection in a state upon request or when the enforcement of federal laws in a state is impractical due to unlawful obstructions or rebellion against U.S. authority.'

Concerning evidence of foreign communication, the Posse Comitatus Act generally prohibits the military from collecting domestic intelligence on U.S. individuals unless there is a connection to a military objective. The Department of Defense is allowed to collect and share information on domestic activities of U.S. persons if those activities threaten DOD personnel or property under limited circumstances.

In summary, the Marines cannot be deployed solely based on evidence of foreign communication for civilian law enforcement purposes. However, the Insurrection Act could provide an exception if such communication is linked to an insurrection or rebellion necessitating federal military action. The military can also gather intelligence related to threats against DOD personnel or property, which might involve monitoring foreign communication associated with these threats. The Posse Comitatus Act's restrictions on domestic law enforcement were clarified to include the Marines, Navy, and Space Force in the 2021 National Defense Authorization Act.

AI response
https://www.brennancenter.org/our-work/research-reports/posse-comitatus-act-explained
https://en.wikipedia.org/wiki/Posse_Comitatus_Act