Asking especially since many of the other major Soviet-run war crimes tribunals were against Nazis and collaborators about war crimes on occupied Soviet territory. Were there plans since 1945 to put captured Kwantung Army officers on trial? If not, what did the Soviets plan on doing with them between 1945 and 1949?
Also, considering the seniority of some of the Imperial Japanese officers being tried (including the Commander-in-Chief of the Kwantung Army himself), the trial was held pretty late in the post-WWII justice process, even after the other Nuremberg trials ended in April 1949. Why did a trial suddenly happen in late 1949? Was the Khabarovsk war crimes trial simply a sop to the newly-founded People's Republic of China, a belated Cold War-era effort to rival the US-led Tokyo Trials, or was it a genuine effort by Stalin to punish war crimes/deter biochemical warfare, etc? And how did the trial affect Soviet relations with China, Japan, the US, and other Pacific nations?
A huge number of the prisoners experimented on by Unit 731 were Russians. There were some prisoners of war in this group, but many also came from the ethnic Russian population of Manchuria. Throughout the Manchukuo period, Russians were systematically discriminated against by Japanese authorities.
The Manchukuo administration and the officers of the local Japanese army - the Kantogun - both consisted of Japan’s fiercest ideologues. The Manchukuo bureaucracy was disproportionately left wing, which was a problem since most Russians in Manchukuo were refugees from Bolshevism and stereotyped by the Japanese as fascists. The command of the Kantogun, meanwhile, was racially conscious and believed in a fundamental racial struggle between Asians and Whites. In short, Russians were discriminated against by two separate organizations with vastly different ideologies, because they occupied an unlucky position as the enemy in both belief systems.
Because of this, up to 30% of Unit 731’s “logs” (their word for test subjects) were Russian in spite of Russians constituting a slim minority of Manchuria’s population.
Your intuition about the trial’s timing being linked to the founding of the PRC is correct - Soviet authorities wanted to court the PRC at a time when they perceived American influence in the East to be growing. However, there was an even more important reason to hold the trials in 1949. The Soviets has dispatched military officers acting under Stalin’s strict orders as “judges” to the Nuremberg and the Tokyo War Crimes Tribunals. They made a complete embarrassment of themselves. One of two Soviet judges at Nuremberg declared before the trials:
We are dealing here with the chief war criminals who have already been convicted, and whose conviction has been already announced by both the Moscow and Crimea declarations.
In Tokyo, the Soviets demanded the inclusion of former Japanese ambassador to Moscow, Shigemitsu Mamoru, in the defendants list, despite having no involvement in war crimes whatsoever. The Soviets rammed through his conviction for “waging an aggressive war” (as ambassador to Moscow) by coercing the tribunal.
The Khabarovsk trials occurred a year after the Tokyo trials, and were essentially the USSR’s attempt to convince the world that its justice system could handle a war crimes tribunal in good order and give the defendants due process.
The attempt backfired badly. The Soviet judges and prosecution rammed through all the convictions within five days. Despite knowing all the convictions to be true, the United States, which had granted immunity to most of Unit 731’s leadership in exchange for their research, condemned the trials as propaganda and fabrication. Because of this, public awareness of the crimes of Unit 731 was low in Japan until the publication of Morimura Seiichi’s expose The Devil’s Gluttony in 1981. The Japanese government had still not admitted the activities of Unit 731 by the end of the 20th century.
Sources:
Materials on the Trial of Former Servicemen of the Japanese Army Charged With Manufacturing and Employing Bacteriological Weapons. <Official Soviet postmortem published for foreign consumption>
Tanaka, Yuki. Hidden Horrors.
Morimura, Seiichi. The Devil’s Gluttony.