 He was in the U.S. Army from 1942 to 1945 where he attained the rank of corporal and won five battle stars.After leaving the Army, Greenberg was associated with the small press publisher New Collector's Group founded by Paul Dennis O'Connor and Hannes Bok.Greenberg and O'Conner clashed about the direction of the press, causing Greenberg to leave, taking the rights to "The Carnelian Cube" by L. Sprague de Camp and Fletcher Pratt as his buyout fee.