Free entrance into National Parks passes are now available to ALL Veterans. (There used to be a minimum of 10% Service Connected Disability for the program.)
Who is eligible to get a free Military Lifetime Pass as a veteran?
For purposes of this program, a veteran is identified as an individual who has served in the US Armed Forces, including the National Guard and Reserves, and is able to present one of the following forms of identification to get the Interagency Military Lifetime Pass when entering a national park:
Unexpired Department of Defense Identification Card (DD Form 2, DD Form 2765, or Next Generation USID replacement)Veteran Health Identification Card (VHIC)Veteran ID CardVeterans designation on a state-issued US driver’s license or identification card
Original source can be found here.