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The entertainment and recreational branch of the U.S. military. The unit was created in 1940 by the War Department with the purpose of enhancing morale, specifically of deployed soldiers, through entertaining performances and leisure programs.
Considering the context in which the term appears in Tony Hillerman's 1980 novel PEOPLE OF DARKNESS, there is a possibility that Hillerman meant Special Forces or Special Operations, and not Special Services. Since the passage in the book discusses explosive mechanisms and ways to conceal them, it is likely that the reference is to the Special Operation unit, which engages in unconventional combat missions requiring highly-skilled personnel.
"Marilyn Monroe performing for troops at a United Service Organization (USO) show in Korea, 1954," photograph by the U.S. Department of Defense, is licensed under Public Domain.
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References
Cooke, James J.
2012 American Girls, Beer, and Glenn Miller: GI Morale in World War II. Columbia: University of Missouri Press.
Monod, David
2005 Settling Socres: German Music, Denazification, & the Americans. Chapel Hill: Univrsity of North Carolina Press.