Quite simply, how can the Soviets have their advance westwards slowed during WWII? What changes, at any level - from a small battle or geographical feature to vast offensives - would cause the Soviets to still be (for example) halfway through Poland, or not even in Poland yet, by the time the Western Allies cross the Rhine or reach Berlin? Any ideas would be really appreciated!
Well...
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Greater German success in 1941, perhaps. For instance, suppose Stalin yields to the Moscow Panic in October, rather than quelling it? He actually went to board a special train at the station - then returned to Kremlin.
If Moscow fell to the Germans, it would not necessarily decide the war in their favor, but it would push the Soviets even further back on the heels, and cripple their later efforts. (Moscow was where the centralized Soviet economy was run from. If all the bureaus and files are destroyed - even if most are evacuated - that's a kick in the head.)
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Better German outcome in the 1942-1943 campaign in the south. Hitler becomes fascinated by Stalingrad immediately, and orders a direct assault early on, before the Soviets dig in there. The city falls, then the Germans sweep out south along the Volga. They still stall out before getting to the Caucasus, but are far less vulnerable to Soviet counterattack. Plus the Soviets are largely cut off from their major oil source. They launch one major attack north of Stalingrad, and force a withdrawal before a Manstein counterattack crushes their lead forces. German reserves are available to parry Soviet attacks against Italian VIII Army and Hungarian II Army, which are weaker due to fuel shortages.
The Axis is substantially better off (and the Soviets worse off) going into
1943. The Soviets regroup and press forward, but not as fast as OTL.
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FDR and Churchill pay more attention to Polish complaints about the USSR's actions in 1939-1940 - including documented proof of the Katyn Massacre. At Teheran, FDR is much more skeptical toward Stalin, and demands that he guarantee the 1939 borders, until plebiscites supervised by third parties confirm any revisions. Stalin first attempts some cynical bullying, then tries to bluster his way out, and FDR decides that from now on, Lend-Lease to the USSR will come after all needs of the U.S., the BCE, Free France, and China have been met - i.e. substantially less than OTL, though still a
lot.
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However... it is unlikely that Soviet forces will be much outside Germany when US/UK forces reach Germany. Because if US/UK forces are getting close, the Germans will transfer forces to meet the greater threat, and Soviet forces will meet less resistance.
OTL... The Soviets were at Warsaw when the US/UK were at the Rhine and Westwall.