The United States Navy

Although the Posse Comitatus Act, which prevents federal military personnel from acting in a law enforcement capacity, applies only to the Army and Air Force, Department of Defense rules effectively require the Navy and Marine Corps to act as if Posse Comitatus did apply, preventing them from enforcing Federal law. The United States Coast Guard fulfills this law enforcement role in naval operations. It provides Law Enforcement Detachments (LEDETs) to Navy vessels, where they perform arrests and other law enforcement duties during Navy boarding and interdiction missions. In times of war, or when directed by the President, the Coast Guard operates as a service in the Navy and is subject to the orders of the Secretary of the Navy until it is transferred back to the Department of Homeland Security. At other times, Coast Guard Port Security Units are sent overseas to guard the security of ports and other assets. The Coast Guard also jointly staffs the Navy's Naval Coastal Warfare Groups and Squadrons (the latter of which were known as Harbor Defense Commands until late-2004), which oversee defense efforts in foreign littoral combat and inshore areas. Additionally, Coast Guard and Navy vessels sometimes operate together in search and rescue operations.
Personnel
The United States Navy has nearly 500,000 personnel, approximately a quarter of whom are in ready reserve. Of those on active duty, more than eighty percent are enlisted sailors, and around fifteen percent are commissioned officers; the rest are midshipmen of the United States Naval Academy and midshipmen of the Naval Reserve Officer Training Corps at over 180 universities around the country and officer candidates at the Navy's Officer Candidate School.