https://codes.findlaw.com/us/title-10-armed-forces/10-usc-sect-772.html

(e)  A person not on active duty who served honorably in time of war in the Army, Navy, Air Force, or Marine Corps may bear the title, and, when authorized by regulations prescribed by the President, wear the uniform, of the highest grade held by him during that war.

https://codes.findlaw.com/us/title-10-armed-forces/10-usc-sect-772.html

(e)  A person not on active duty who served honorably in time of war in the Army, Navy, Air Force, or Marine Corps may bear the title, and, when authorized by regulations prescribed by the President, wear the uniform, of the highest grade held by him during that war.