2017-07-15 17:04:33 UTC
I think, unless provoked badly by Japan, the Soviet Union is probably going to keep its own powder dry till the 1950s or later. It would have reason to fear a powerful anti-Soviet coalition if it tries to revise any of its borders in Europe or the Middle East. Finland, Poland, Romania and Turkey are all substantial countries. Even the Baltic States would likely be safe from invasion through 1950 and beyond, because grabbing one of them still could cost more than it is worth in terms of generating possible backlash by larger powers.
After about 1950 the situation probably gets more permissive for Soviet aggression in Europe. By 1950, the Soviets will be *much* more industrialized, having gone through four and a half five years plans, and under mid 20th century conditions this is probably going to widen the Soviet advantages against its immediate western neighbors, and even Germany, quite a bit. Another factor potentially making the environment for Soviet aggression more permissive later in the 1950s or 1960s could be the development of atomic weapons and delivery systems.
Even if, as is likely, the USSR is not the first atomic power, and the US, Britain, France and Germany all achieve that status before the Soviets, the Soviets going to be far ahead of any of their immediate western neighbors. An atomic shield, in an era of relatively unilateral-ized military policies and no NATO-style guarantees, could embolden Moscow to use brute force to achieve some revisionist territorial aims.
Alternate conditions besides WWII that could prompt the Soviet Union to expand its territory in the 1940s could include grabbing Bessarabia if Bulgaria and Hungary end up at war with Romania, without German participation. Or limited gains could be made against Poland in the context of a Polish-Lithuanian War, Polish-Czech war, or war amongst the Baltic states, or Turkish involvement in war against another great power. Of course gains against Poland are possible in the event of a less than total German-Polish war that does not involve other major powers. A successful revolutionary movement or disorder so extreme in a neighbor that makes a modest military fait accompli look super easy could also tempt Stalin.