32 U.S.C. § 315

(a) The Secretary of the Army shall detail commissioned officers of the Regular Army to duty with the Army National Guard of each State, the Commonwealth of Puerto Rico, the District of Columbia, Guam, and the Virgin Islands. The Secretary of the Air Force shall detail commissioned officers of the Regular Air Force to duty with the Air National Guard of each State, the Commonwealth of Puerto Rico, the District of Columbia, Guam, and the Virgin Islands. With the permission of the President, an officer so detailed may accept a commission in the Army National Guard or the Air National Guard, as the case may be, terminable in the President’s discretion, without prejudicing his rank and without vacating his regular appointment. (b) The Secretary of the Army may detail enlisted members of the Regular Army for duty with the Army National Guard of each State, the Commonwealth of Puerto Rico, the District of Columbia, Guam, and the Virgin Islands. The Secretary of the Air Force may detail enlisted members of the Regular Air Force for duty with the Air National Guard of each State, the Commonwealth of Puerto Rico, the District of Columbia, Guam, and the Virgin Islands.