In my opinion USSR taking Manchuria on T1 is practically a death warrant for Russia. Why?
1.) You would almost certainly take heavy losses in INF before you prevail.
2.) Japan will be forced to take back Manchuria on T1. This will indeed slow down Japan’s advance in East Asia for a turn, but the tradeoff is not worth it because…
3.) USSR is forced to either leave the rest of their Eastern territories underdefended, or to commit signifigant forces from the West to shore them up. Either way you’ve weakened USSR far more than the measly 3 IPC gain for 1 turn or the setback of Japan are worth.
CAVEAT:I suppose with careful follow-up planning by all the Allies, the situation could be exploited for a possible edge on Japan. A case in point would be if all the Allies gang up on Japan T1 with concerted attacks, or if the UK wish to place an IC on India. The USSR siezure of Manchuria could then prove valuable to the overall Allied picture. But in general I believe it would be far more useful for USSR, if he/she is feeling aggressive, to hit-and-run Manchuria by scoring 1 or 2 hits & retreating with whatever’s left, than to actually take the territory & set in motion a chain of events that would be detrimental to the Allied cause…
Just my opinion…
Ozone27