I think one major thing many people do not fully understand is that the Bolsheviks did not have a firm grasp over the borders they claimed to hold, and that people see on a map as being under USSR/Party control. It was not really until after Stalin's collectivization of the peasants that the Soviets had full control over their own territory. Basically, the Communists ruled the major cities, but Stephen Kotkin points out in his biography of Stalin that many peasants did not even know whom Stalin was in the 1920s. The revolt in Krondstat and Tambov threatened Bolshevik power internally. Krondstat forcing the passing of the NEP. aka market concessions from die hard Socialists. Germany almost ousted the revolution with a small army towards the end of WWI. The Japanese occupied their eastern section with almost impunity during the civil war. This is not even mentioning the civil war. The USSR felt as if it were under a constant state of siege all throughout the revolution and 1920s

There was aperiod in the mid 1920s where Stalin was still gaining complete control over the party. The party engaged in plots with powers they were trying to trade with, and get loans from. The USSR egged on a hopeless uprising in Germany, one that was doomed to failure. Also, they supported Britain's first nation wide general strike, while also trying to negotiate with the Brits for trade. The Chinese communists were massacred, mostly due to Stalin's insistance that they stay allied to Cheng Kai Shek. and other countless plots. There was always hanging onto the hope of the coming world wide revolution, but Stalin knew they were in for the long haul, thinking that they would need to wait for another Capitalist word war to spark it.

Stalin's goal was Socialism in the USSR, and he pursued that to the end. The Bolsheviks expected their revolution to spark an immediate worldwide uprising, but when that did not happen, and Poland defeated them, they fought just to reign in their own territory. Stalin went on to focus internally, much to Trotsky's dismay and then proclaiming that Stalin was "the grave digger of the revolution." Stalin focused internally because he understood the Nationalist and peasant movements happening throughout the USSR would need to be reckoned with. More than that, the USSR really had no hope in engaging in open warfare with modern developed powers.

The USSR was extremely vulnerable in its early years. It was not until the late 1920s that production even reached ten percent of its pre-war military production. By contrast, Britain was 50% higher and America 75% higher than their respective pre-war productions at that same time. Moreover, Stalin was dead set on destroying the parallel peasant revolution, and collectivizing the farms there. In the early 1930s, the nation that previously fed Europe was starving due to the de kulakization (killing rich farmers) and collectivization. This was right as the USSR started to really get its heavy industry going by exporting grain for machinery, during a famine. Soon after, the USSR's production of military would start to pick up, but so would Germany's, precluding any kind of preemptive invasion and Stalin would cripple his military.

Also, the great purge was a significant event that wiped the Red Army's fighting capability. A majority of generals were executed in a supposed Trotskyite plot to depose Stalin. It cannot be understated the delicacy in which the USSR's state was right up until Hitler invaded. The USSR showed its lack of capability in the Winter War with Finland. What they did produce was shoddy, and used outdated tactics by a political military with no experience. Really, it was the majority view that the USSR would crumble if the door was kicked in. Everyone underestimated its production and war making capabilities because the reputation had been well earned years prior.

I recommend Stephen Kotkin's work. His soon to be three volume biography of Stalin is a great read by a geopolitically focused professor and historian.

You might be surprised, but it could be argued they made such an attempt. They were stopped by Poland in 1920, which is most definitely before the rise of Nazi Germany.

To get a little more technical, the Soviet Union hadn't quite formed by then. It was Soviet Russia and Soviet Ukraine, and they were only really figuring everything out. Originally, the idea was to spread the Communist revolution globally, and a relatively small Western neighbour that had been a part of Russia for over a hundred years seemed like a perfect opportunity to do just that.

They proceeded with the attack, and the Western powers soon assumed that Poland was doomed. Nevertheless, the Polish Army managed to pull off the Miracle of the Vistula and successfully repelled the Soviets. This hit the brakes on their invasion and forced them to focus inwards; the Soviet Union was properly formed in 1922.

Later on, the ideology switched to Socialism in One State, as devised by Stalin. After the actual Soviet Union formed, they were focused on implementing their ideology, and then a massive power struggle erupted after Lenin's death. Once Stalin came out on top, he conducted purges that touched pretty much everything in the country. However, by the late 1930s, he managed to consolidate power. The Spanish Civil War and co-operation with Germany contributed greatly to new military developments and Stalin could finally build up a massive war machine.

Of course that policy didn't exactly stop the Soviets anyway. It's even argued that it was an interim policy and that it was just a pragmatic stopgap solution before a strong Soviet Union could bring about a global revolution. As time went on and Hitler rose to power, the power struggles in the Soviet Union ended with Stalin coming out on top, and they started making plans for a comeback. Now, this is a little out of your time frame, but during the run-up to WW2, the Soviets made their own plans to expand westwards. Once they secured the Molotov-Ribbentrop pact, they made moves to expand westwards. Having made a grab for as much Polish territory as they could before the Germans reached them, they attacked Finland and occupied the Baltic states in the 1940s.

And here's the kicker: there's actually serious debate as to whether Hitler didn't just beat Stalin to the punch. I've seen plenty of articles mention that the Soviets were fully intending to attack Germany once built up sufficiently; I can't really source this very well, though.

This is generally textbook knowledge in the area.

The basic takeaways to me seem to be that if you squint hard enough, you could say that it's because they tried and got beaten. If you want to be a bit more strict about the Soviet Union, the lack of conquest attempts was mostly due to the fact that they needed to organise themselves and resolve the power struggle after Lenin's death. They stabilised sufficiently roughly at the same time as Hitler rose to power and then started a parallel buildup in military capability that ultimately helped crush the Germans. Of course the defeat of 1920 was also a factor that influenced the Soviet Union early on, but the instability and the power struggle became a much more decisive factor in preventing an invasion of Europe.

You should be able to find a little more detail in Norman Davies' White Eagle, Red Star: the Polish–Soviet War, 1919–20 or Adam Zamoyski's Warsaw 1920: Lenin's Failed Conquest of Europe. There's also God's Playground vol. 2 by Davies which covers the Ribbentrop-Molotov pact to some degree. /u/hamiltonkg also covers many related points.