Seeing as how the USSR declared war on Japan between the atomic bombings of Hiroshima and Nagasaki, just days before the capitulation of Japan, how did Japan ever manage to collect a significant number of Soviet POWs?
Regards,
The Russians and Japanese had been in conflict on and off directly and by proxy since early 1900s. In the 1930s when the Japanese were heading West through China toward the Soviet border, it came to a head but both empires were distracted on other fronts for a full commitment of resources.
Battle of Lake Khasan took it to WW2 levels before a formal declaration of war.