There were several Geneva Conventions prior to the 1949 one; the "Convention relative to the Treatment of Prisoners of War" was signed in 1929.

https://ihl-databases.icrc.org/en/ihl-treaties/gc-pow-1929

The current Geneva Convention was in 1949, but the original Geneva Convention dates back to 1864.

Of course, the previous poster is noting that the USSR blithely disregarded the conventions for others' POWs as well as its own: most returning soldiers were considered traitors to the Motherland and summarily executed or sent to the Gulag to die more slowly.