From
Stalin's Letters to Molotov, 1925-1936 (dated October 7, 1929--this was during the Sino-Soviet conflict caused by Zhang Xueliang's attempt to gain control of the northern part of the Chinese Eastern Railway):
"There will be a lot of trouble with China. By the way, I think it's time to think about organizing an uprising by a revolutionary movement in Manchuria. The isolated detachments being sent to Manchuria to perform isolated tasks of an episodic nature are a good thing, of course, but they are not enough. We have to go for bigger things now. We need to organize two double regiment brigades, chiefly made up of Chinese, outfit them with everything necessary (artillery, machine guns, and so on), put Chinese at the head of the brigade, and send them into Manchuria with the following assignment: to stir up a rebellion among the Manchurian troops, to have reliable soldiers from these forces join them (the others should be sent home after removing the officer corps), to form into a division, to occupy Harbin, and, after gathering force, to declare Chang Hsueh-liang overthrown, establish a revolutionary government (massacre the landowners, bring in the peasants, create soviets in the cities and towns, and so on). This is necessary. This we can and, I think, should do. No 'international law' contradicts this task. It will be clear to everyone that we are against war with China, that our Red Army soldiers are only defending our borders and have no intention of crossing into Chinese territory, and if there is a rebellion inside Manchuria, that's something quite understandable, given the atmosphere of the regime imposed by Chang Hsueh-liang. Think about it. It's important."
https://books.google.com/books?id=jjzopxC9ICYC&pg=PA182
This obviously would have gone much farther than what the USSR did in OTL, which was basically simply to use force to restore the 1924 Soviet agreement with Zhang Xueliang's father. Lars T. Lih notes in his preface that "This revolutionary daydream seems atypical of the cautious Stalin, and there is no indication anything was done with it."
https://books.google.com/books?id=jjzopxC9ICYC&pg=PA35 Was there any chance that Stalin would actually follow through with this? It sounds like a recipe for a war with Japan, in which Japan would not only have the support of the other major capitalist powers, but (unlike what it did in Manchuria in 1931 in OTL) could claim it was acting in
defense of China.