I *think* it might be from the National Security Act of 1947, which broke the Army Air Corps into its own Air Force, and established the Department/Secretary of Defense. Also where CIA was established.
The Marine Corps is technically under the Department of the Navy. However, I think this wording is what has people saying the Marine Corps reports directly to the President:. I did not find this wording under the duties of the other branches of the military (and shall
perform such other duties as the President may direct):
(c) The United States Marine Corps, within the Department of the Navy, shall include
land combat and service forces and such aviation as may be organic therein. The Marine
Corps shall be organized, trained, and equipped to provide fleet marine forces of
combined arms, together with supporting air components, for service with the fleet in the
seizure or defense of advanced naval bases and for the conduct of such land operations as
may be essential to the prosecution of a naval campaign. It shall be the duty of the Marine
Corps to develop, in coordination with the Army and the Air Force, those phases of
amphibious operations which pertain to the tactics, technique, and equipment employed
by landing forces. In addition, the Marine Corps shall provide detachments and
organizations for service on armed vessels of the Navy, shall provide security
detachments for the protection of naval property at naval stations and bases, and shall
perform such other duties as the President may direct: PROVIDED, That such additional
duties shall not detract from or interfere with the operations for which the Marine Corps
is primarily organized. The Marine Corps shall be responsible, in accordance with
integrated joint mobilization plans, for the expansion of peacetime components of the
Marine Crops to meet the needs of war.
https://global.oup.com/us/companion.websites/9780195385168/resources/chapter10/nsa/nsa.pdf page 9
That explains that. But could they be switched to federal Marshall’s statice. I know when the county over was having problems with drugs and aids the federal marshals came in. Local Leo were corrupt. Many of the Feds has military background.
Thanks!