It looks as if the poem is contained in the book Our Fight: Writings by Veterans of the Abraham Lincoln Brigade, Spain, 1936-1939 edited by Alvah Cecil Bessie and Albert Prago (New York: Monthly Review Press, 1987), pages 237ff.

Because of copyright, it's classified as "snippet view only" by Google Books, but I was able to tease out this much information:

"Theodore R. Cogswell was born in 1918 in Coatesville, Pennsylvania. He spent thirteen months in Spain, serving as an ambulance driver. On his return he enlisted in the US army air corps and served in the China-Burma-India theater. He then returned to university and earned a BA and MA in English. He has published two collections of short stories, a science fiction novel, and was national secretary of the Science Fiction Writers of America."

Worldcat.org might help you find the book in a library near you if you go to that page and enter your Zip Code or postal code.

See also this list of books by Theodore R. Cogswell.