Complex sentence: In May 1925 a Junkers F.13 entered service as the first aircraft in Mongolian civil and military aviation and in march 1931, the Soviet Union donated three Polikarpov R-1s to the Mongolian Peoples Army, with Mongolia purchasing a further three R-1s.
Split into simpler sentences: In May 1925, a Junkers F.13 entered service as the first aircraft in Mongolian civil and military-related aviation. In March 1931, the Soviet Union donated three Polikarpov R-1s to the Mongolian People's Army, with Mongolia further purchasing three R-1s.