Final answer:
The Posse Comitatus Act of 1878 prevents the U.S. Military from engaging in the enforcement of state civil or federal laws, ensuring civilian law enforcement is responsible for maintaining civil law and order.
Step-by-step explanation:
The law that makes it illegal for the U.S. Military to assist in the enforcement of state civil or federal laws is the Posse Comitatus Act of 1878. This act prohibits the use of the U.S. Army and Air Force to execute the laws unless expressly authorized by the Constitution or an act of Congress. Furthermore, the 1956 revision included the U.S. Navy and Marine Corps, while the U.S. Coast Guard is exempt as it is part of the Department of Homeland Security and can be involved in law enforcement during peacetime. The purpose of the Posse Comitatus Act is to limit direct military involvement in civilian law enforcement activities ensuring civilian authorities are responsible for the conduct of civil law and order.