How could I get copies of military records on forebears who served in foreign armies?

Question

I'd like to get copies of the service records of my great great grandfather who served in WW1 in the Russian Imperial Army. After reporting for duty, he was sent to the German Front. He wrote one letter to his wife (my great great grandmother). After that he disapeared, never to be heard from again. One generation later there is a family story that my great grandfather (his son) was living in Petrorad. He was a Party Member and happened to run into a man from his village who served with his father. The other man said that my great great grandfather and this man said a Jewish Prayer commonly said when one feels he is close to death, left the trenches and ran towards the German line. Everything became choatic and the man said when it was over, bodies were everywhere and he never saw him again. He was unable to confirm if he died and the man was just glad to have survived himself.

Answer 1

There are no records in great detail from that period, during the civil war and immediately after the communists came to power many official documents were destroyed.