The Geneva Convention on Prisoners of War was signed at Geneva, July 27, 1929. Its official name is the Convention relative to the Treatment of Prisoners of War. It entered into force 19 June 1931. It is this version of the Geneva Conventions which covered the treatment of prisoners of war during World War II. It is the predecessor of the Third Geneva Convention signed in 1949.
On their web site, the International Committee of the Red Cross states that:
During World War II, Soviet prisoners of war (POWs) held by Nazi Germany and primarily in the custody of the German Army were starved and subjected to deadly conditions. Of nearly six million who were captured, around three million died during their imprisonment.
In June 1941, Germany and its allies invaded the Soviet Union and carried out a war of extermination with complete disregard for the laws and customs of war. Among the criminal orders issued before the invasion was for the execution of captured Soviet commissars. Although Germany largely upheld its obligations under the Geneva Convention with prisoners of war of other nationalities, military planners decided to breach it with the Soviet prisoners. By the end of 1941, over 3 million Soviet soldiers had been captured, mostly in large-scale encirclement operations during the German Army's rapid advance. Two-thirds of them had died from starvation, exposure, and disease by early 1942. This is one of the highest sustained death rates for any mass atrocity in history.