Thanks! So I was partly correct perhaps, with the Soviets wanting to spare themselves more loss. I am a bit surprised how large an offensive they were able to mount. Do you know if they had maintained a large army in the east as a defense against a possible Japanese invasion - or did they need to bring over large amounts of troops from Europe?
I’m pretty sure it was the latter.
The answer is "yes." Of course, it depends on when you are looking at the Soviet strength in the East. Before June 1941, the USSR kept a large army in the Far East because of the 1939 clashes with the Japanese at Nomonhon/Khalkin Gol. But in the fall, when Soviet spy Richard Sorge confirmed the Japanese were going south, not north, Stalin was able to shift about 30 well trained and well equipped divisions from the Far East for the Battle of Moscow.
Thereafter, the Soviets rebuilt their Far Eastern Military District. Command was given to Maxim Purkayev, who had commanded Kalinin Front against the Germans in Fall 1942-winter 1943. He was sacked as a result of the failed Operation Mars, but was still a fairly capable commander. While the Soviet forces were never on a par with what was committed in the west, from 1943 on, the Far East Military Districts were well manned and relatively well equipped. The Soviets always feared a Japanese intervention.
So while the bulk of the troops used against Japan were already there, the Soviets did augment them with considerable forces from the west, most notably 6th Guards Tank Army, which fought in Ukraine and the Balkans. This unit formed the mobile shock spearhead of the Soviet forces hitting Manchuria from the western flank through Mongolia. That sector had such poor roads and limited rail net, the Japanese were totally surprised when the T34s came out of te desert.
The troops invading Manchuria were almost all brought from Europe. The reason Stalin remained neutral with Japan is that he could not afford a two front war. Now that he won in Europe he wants to grab what he can in Asia.