If the Soviets get involved, it'll be a Continuation War rather than an invasion of Eastern Europe - and the desire for a buffer zone in Eastern Europe was in any case driven by Barbarossa in the first place. No Barbarossa, no real worry about buffer zones.
It's not so simple. Barbarossa certainly heightened Soviet desire for a buffer zone with the west but Soviet/Russian ideas of buffer zones is a much older one in history... indeed, the entire history of Russian expansionism back to the Duchy of Muscovy has basically been one long search for frontiers and natural boundaries which offer buffer zones that shield their heartland from invasion. By the start of the 19th century they had achieved this to their south (via conquering the Caucasus), east (via conquering Siberia and the Central Asian Steppes up to the mountain ranges, deserts, and mountains there), and southwest (via conquering up to the Carpathians). But that still left them vulnerable to an attack from directly west across the North European Plain and indeed, they were invaded or threatened with invasion from this direction multiple times even before 1941. This video offers a good synopsis of the issue. Throw on top of this the fact that the Soviets believes that eventually the capitalist powers will try to destroy the USSR, and indeed that the Anglo-French had made an attempt to destroy the Bolshevik in it's revolution in its infancy (at least from the Soviets viewpoint) during the Russian Civil War, and it's easy to see how the Soviets may still seek to put another country or two as a puppet across this region between them and the western powers.
In addition to the above geopolitical issue is an ideological one as well. While Stalin did prove to be more of a pragmatic-realist in his foreign policy dealings he was still also ideologically a committed Leninist and that likely did play a significant part in his decision to establish client states across Eastern Europe as a buffer zone IOTL. Because in Marxist ideology the means bring about the post-capitalist Communist order is to revolutionize capitalism in its strongholds, to take the concentrated powers of production of the most highly developed capitalist societies and socialize them. There was something of this ideological motivation in the failed 1921 counter-invasion of Poland (as well as to punish the Poles for daring to invade Soviet soil, of course) and while it was downplayed by Stalin in the 1930s with socialism in one country, it still was very much lingering around as a long-term goal. To quote Norman Davies...
No Simple Victory: Europe at War said:The region of Europe that lies between Moscow and Berlin, sometimes called East-Central Europe, has never been well known to Westerners. But the observant reader may have noticed that the area of the Bolsheviks' dashed internationalist hopes, to which they would some day return, coincided very closely with the area of Hitler's projected Lebensraum. Even in the 1920s or 30s, a prescient analyst might well have spotted where the next great European clash of arms might be concentrated.
So there are still good, solid reasons as to why ITTL Stalin might back-stab a collapsing Germany at the very end to take Poland and Germany up to the Oder.
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