>>12365560 lb/pb questions about USMC and ANG
What authority does POTUS have specifically over the Marines?
The Marines are the only branch of service that falls directly under executive command
10 U.S. Code § 8063 - United States Marine Corps: composition; functions
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. However, these additional duties may not detract from or interfere with the operations for which the Marine Corps is primarily organized.
Focus on the power of POTUS as it relates to the Marines.
The National Security Act of 1947, amended in 1952, specifies the duties of the USMC to include: "perform such other duties as the president may direct."