Folks in the US honor Veterans this weekend. While we're on the topic, lets not forget those Veterans of the Psychic Wars. The first reference of the term appears to be in Jim Morrison's poem Far Arden, 1969 (ref starts at 2 min 50):

A constant theme in Michael Moorcock's Elric novels (1972) who worked with Hawkwind (I consider them the Grateful Dead of England) and used the term in this "song" in 1975: Standing on the Edge (at 1 min 45)

Michael Moorcock of course also collaborated with Blue Oyster Cult in 1981 on the immensely popular song of the same name: