MLA

Service, United States. Army Map. Divisions of the Union of Soviet Socialist Republics as of September, 1939 / Compiled and Drawn by Army Map Service. The Service, 1942. https://collections.library.yale.edu/catalog/15474758.

APA, 6th edition

Service, U. S. A. M. (1942). Divisions of the Union of Soviet Socialist Republics as of September, 1939 / compiled and drawn by Army Map Service. [Maps]. The Service, https://collections.library.yale.edu/catalog/15474758.